I think that one might be an exception rather than the rule. Kingdom Battle was still a Switch exclusive, after all - it's not like Ubisoft has a console that competes with Nintendo.
Generally, I don't see Nintendo announcing a Smash character at another conference, unless somehow Smash Ultimate was coming to the Xbox. Otherwise, Microsoft would gain very little from the announcement. If it was, say, a joint game project between Big N and Microsoft, that was available on both consoles, then yeah, maybe. Smash? I don't see it happening.