I am guessing that *.itch.io points to some IP and that subdomains are resolved on the app level. Many services do that (my god forgotten app does, Tumblr does, I am sure others must, too)
It's a CNAME for itch.io, but the application doesn't resolve DNS. DNS does. If the DNS spec says that underscore isn't valid, then presumably we would never even get to the application level because the hostname wouldn't resolve to itch.io.
I guess it depends on how the DNS server deals with the wildcard domain. I can tell you that djbdns doesn't care about the part covered by the wildcard, it will happily resolve _!test.domain (although it is an invalid name according to the RFC).
