Egregious Longhorn fan here... given that a normal stance with legs spread is just shy of 3 feet wide, we should be looking to see if his right foot moved significantly past where his left foot started.

And as much as I don't want to say this... I think it's clear he did.

Let Me Whip Out My Slide Rule ...
 

NFHS 4-42-6: The designated throw-in spot is 3 feet wide with no depth limitation
and is established by the official prior to putting the ball at the thrower’s disposal.
NOTE: The thrower must keep one foot on or over the spot until the ball is released. The
traveling and dribbling rules are not in effect for a throw-in.

Lets' say that the ball is handed, or bounced, to the inbounder, who, for sake of argument, has his feet eighteen inches apart while standing in the center of the three foot wide designated spot. Let's say that he wants to move left. He can move both feet nine inches to the left with no restriction (he's still in that three foot wide spot). Now he has to keep one foot in that spot (let's say he keeps the right foot there) but he still may move the left foot as far as he can without moving the right foot (still in the spot). He can probably take a three foot stride to his left, so in total he has moved about three and a half feet to his left, all legal. As long as he keeps one foot over that three foot wide designated spot, his other feet can move, left to right, legally, in an, almost, seven foot wide area.

Picture a really tall kid in one of the free throw lane spaces. As long as he keeps on foot in that three foot wide lane space, he can probably take one stride left, or right, and almost touch the adjacent lane mark. That's more than nine feet, including the marks themselves. So an average size kid can easily legally "cover" seven feet (left to right), as long as he keeps one foot in that three foot wide area.