Foul ball w/ 2 strikes - slow pitch

[BoomerSooner]

I saw the other foul ball with 2 strikes thread and it reminded me of a comment I heard a couple of weeks ago that I just dismissed at the time but then put some more thought into after the fact.

With less than 2 outs and 2 strikes, the batter hits a long fly ball that is two-thirds of the way down the left field line (from the bag to the fence). The left fielder gets to the area where the ball is coming down and is tracking the ball and setup to make the catch, but at the last second moves to let it drop foul. The call was foul ball, batter is out, but otherwise it is treated like any other foul ball (ball is dead, no runners can advance). The comment that was made was that if the fielder intentionally lets a foul ball drop, the runners could still advance on a foul ball in that situation. Like I said, I just dismissed the comment since it seemed like a player making a silly argument so that his teammate at 3rd could have been able to tag up and score (which he would have easily had the ball been caught). I did later wonder if there ever was something like this that was allowed.