What if there are 65535 active send requests? Perhaps of which 65534 are opened by an attacker?
Yes, an attacker can denial-of-service by guessing once on all the (possible) requests. There is some work towards stopping this sort of behavior (see e.g. https://github.com/warner/magic-wormhole/issues/126)
EDIT: was just explained in other comment by meejah, thanks! (https://news.ycombinator.com/item?id=14649727#14650551)