Here’s how the logic makes sense to me:

A late hit is a hit after the play is dead. If he were to have slid, stood up, walked towards the sideline, and then been tackled out of no where, that still would’ve been a late hit and I don’t think anyone would say Clemson should get a 1st down for something that happened so long after the play.

So since there is a “too long”, you have to have something thay designates when is too long. And for this, it’s once the play is dead, that’s the description of the penalty.

And for this play, it was dead as soon as he began his slide, by rule. And as soon as the play was dead, it became Duke ball.

And I don’t think there’s anything wrong with that. Committing a penalty after a play is over shouldnt change how that past play worked. Otherwise, it’s like the rule NFL used to have where excessive celebration after a TD could take away the score