Dislodging the Ball from the Thrower's Hands, but...
 

Is the actual element of "reaching through the boundary plane" necessary to issue a 4-47-3 warning along with a player technical in this play?

A1's momentum carries him OOB after a drive and successful score. B1 grabs the ball and steps OOB to initiate the fast break with a quick throw in. On the way back to the court, A1 makes contact with the ball and dislodges it from B1's hands. A player technical was issued. But should this have been a player technical with the delay of game warning at the same time as dictated by 9-2-10 PENALTY 3, 9.2.10A, and 10-4-10? Those rules all include the feature of reaching through the OOB boundary plane, which this player did not do because he was already OOB.

The coach asked, "Don't we get a warning for that?" I said, "Coach, when your player dislodged the ball and received the technical foul, that was also considered the warning," recalling in my mind the above rule-based situations. I'm not sure if I was right or not. Any help?

Yes it is a warning and a T. Why does that matter? Well any other warning is not a T because you did this earlier in the game. So you were right to address this to the coach.

Peace

Interesting that 10-4-5a allows us to issue a player T but without the "reaching through" aspect and without the otherwise associated delay involved. I would consider that an alternative, but it seems as though the situation merits the warning.

Just did a whole bunch of reading. 10-4-5a has the phrase "or from being put in play" which is not included in any of the rules or references that allow for a delay warning.

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By literal rule, I'm tending in this direction of not issuing the delay in conjunction with the player T for delay of game per 10-4-5a, though it seems that what the player did was more egregious than "reaching over the boundary line" to contact the ball in the thrower's hands. An anomaly, perhaps. I can see selling it either way. Glad it's rare.<iframe width="560" height="315" src="https://www.youtube.com/embed/3l5z4NLAmQM" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

Not Exactly The Same ...
 

... but will this provide any guidance?

2011-12 NFHS Basketball Rules Interpretations
SITUATION 8: Team A has a designated spot throw-in along the end line. Thrower A1 extends the ball with his/her arms over the end line such that part of the forearms, hands and the ball are entirely on the inbounds side of the boundary line. B2 slaps A1 on the wrist and dislodges the ball. RULING: When a defender makes contact with a thrower-in, the result is an intentional foul. Where A1’s arms are located (on the inbounds or out-of-bounds side of the boundary line) is immaterial for this penalty to be assessed. A1 is awarded two free throws and Team A awarded a throw-in at the spot nearest the foul. COMMENT: For a boundary-plane violation warning to also be assessed, the defender must actually violate the rule and penetrate the boundary plane. (4-19-3e; 4-47-1; 7-5-4b; 9-2-10 Penalty 4)

Ooooooo. Yes, I ran across that in the Interps. In fact, that also made it into the Casebook, 9.2.10C. However, I didn't associate it at the time with the play under discussion. If the NFHS is stating a universal principle there, this certainly would apply and would suggest that a 4-47-3 DOG warning NOT be issued for this situation.
Thanx.

Let's Go To The Videoptape ...
 

I think it does suggest a DOG warning would be issued.

Being out of bounds is completely penetrating the OOB plane.

I'm not sure this is true. Being on the other side of a plane doesn't mean one is penetrating it.

Penetration ...
 

A well thought out opinion, but check the wording, the defender had already penetrated the boundary plane well before the thrown started, he didn't penetrate it (from inbounds to out of bounds) during the actual thrown.

Other than strike the ball from the inbounder's hands, thus delaying and interfering with the throwin (for which he was charged with a technical foul), he did nothing wrong, his momentum carried him off the court, it is not a violation for a player to unintentionally leave the court, and after being legally out of bounds he was expected to not delay in returning back inbounds.

10-4-10: A player shall not: Reach through the throw-in boundary-line plane and touch or -
dislodge the ball as in 9-2 Penalty 3.

9-2-10-Penalties:
1. The first violation of the throw-in boundary-line plane by an
opponent(s) of the thrower shall result in a team warning for delay
being given (one delay warning per team per game). The warning does
not result in the loss of the opportunity to move along the end line
when and if applicable.
2. The second or additional violations will result in a technical foul
assessed to the offending team.
3. If an opponent(s) reaches through the throw-in boundary-line plane
and touches or dislodges the ball while in possession of the thrower or
being passed to a teammate outside the boundary line a technical foul
shall be charged to the offender. No warning for delay required.
4. If an opponent(s) contacts the thrower, an intentional personal foul
shall be charged to the offender. No warning for delay required.

9.2.10 SITUATION A: A1 is out of bounds for a throw-in. B1 reaches through the boundary plane and knocks the ball out of A1’s hands. Team B has not been warned previously for a throw-in plane infraction. RULING: B1 is charged with a technical foul and it also results in the official having a team warning recorded and reported to the head coach.

10.4.10 SITUATION C: Team A scores near the end of the fourth quarter and is trailing by one point. B1 has the ball and is moving along the end line to make the throw-in. A2 steps out of bounds and fouls B1. Is the foul personal or technical? RULING: This is an intentional personal foul. The time remaining to be played or whether Team A had been previously warned for a delay-of-game situation is not a -factor. If the team had not been warned, the foul constitutes the warning. (4-19-1; 9-2-10 Penalty 4)

I would not use the case play to address the OP. The end result might be the same, but I think the highlighted part of the case play is meant for the (normal / usual) situations where B is inbounds and A is out of bounds.

Abnormal, Unusual, Rare ...
 

Freddy certainly has a propensity for coming up with abnormal, unusual, and/or rare situations.

like

Never seen Freddy's play, but I have seen a player whose momentum carried him OOB on a made lay-up reach through the boundary line in the opposite direction in an attempt to deflect the throw-in. What would we have if the player who was already OOB does tip the throw-in?