When closing the throttle, a lot of the fuel on the walls of the manifold
(and there is a LOT of fuel on the walls) will flash off.

That's very true for a cold engine and especially with autofuel (which
I use in the 172M). In this case he was using 100LL in a warm engine,
so it wouldn't seem so likely.

I have never heard before of the kind of stumble given by the OP. It
could be a rich flash afterall.