You're right, there is no guarantee. Just that kernel folk aren't entirely opposed to the idea, or I think we'd heard about it by now.

I think what convinced me at least was the argument that larger codebases like IPSec were merged into the kernel, so why would WireGuard have any trouble?

Of course someone upstream might just decide to say no, and we'll have to live with the kernel module which is just as fast especially since it tends to reach line rate.