I doubt anyone thinks that Nole choked, but if we go back to Tignor's comment that he "looked constrained, more hopeful than aggressive. He didn’t fire himself up, rev the crowd, or try to take the match to his opponent until deep into the fourth set," we could probably agree that in the second set, just when he would be expected to hunt Stan down, and press home his advantage, his level dropped, while Stan became emboldened.
Is this "choking"? I wouldn't call it that, I'd say it was being more conservative than he needed to be. If it was a semi, I imagine he'd have been more aggressive. Stan rose then and seized the momentum, so it's a chicken and egg thingy. Stan's response to losing the first set was incredible.
But "choking" would be more like Novotna, or McEnroe in 1984, where he was dominating a player who he'd dominated for a long while, up two sets, up a break in the fourth - and he bottled it. Novak was actually scrapping quite hard out there, but the momentum swung against him.
The issue of the dropshots was to highlight just how lost he was in the face of the assault...