Of cours it wasn't a choke. A choke is a missed penalty. Serve & volley on matchpoint is the opposite of a choke, even more on second serve. It's risky but brave and put pressure on the opponent. However Fed deals rather good with pressure and was astute enough so make Stan volley, even with some floating wicked slice on the one that seemed most easy for Stan to put away. Overall Wawrinka got a little tense when serving for the match but no choke whatsoever.
Not sure what "A choke is a missed penalty." means, but, to me, a choke can be physical or mental, but one result is, you try to do something that you can normally do frequently under "normal" conditions, but, because of the importance of the occasion, you don't execute-and that can be mental. I like Stan, and I feel bad for him-nor do I like serve and volley, which I have long enjoyed, getting "punished" in a big spot 3 different times. But the fact of the matter is, # 1, he was doing something he doesn't normally do, and, if you make it, it's courageous, if you don't, you're gonna face questions, and, # 2, on 1 of the match points, the one where he served and volleyed on the second serve, THAT volley is one I'm confident he would make, say, 80% of the time, it wasn't that hard, but he absolutely, positively, choked on it, IMO.