I don't think it's impossible Nintendo would announce Steve on Minecraft's birthday, with Mojang putting the news on their website along with everything else, but yeah it's probably very unlikely, since Nintendo would probably want to wait 'til E3 or something and they may see it as Smash overshadowing the other news even though it's still technically Minecraft news as well.
However, I don't see much of a problem other than some people not recognizing the anniversary itself (admittedly I did this myself before correcting my mistake as a post on a Smash site leading me there leaves me with Smash hype), especially since some are probably younger and are definitely part of both fanbases.