Of course he could be revealed in the Direct. We have no credible leaks otherwise(the only leak we have is a Sora one, and it has no real credibility just a decent believable thought behind it and a good guessed date).
The problem is, people give too much credit to E3. Steve was never going to be at E3 as long as Min Min was delayed that long, after all. Steve even then at best had a short delay cause it would sell even better by being revealed near Minecon, gathering more attention. Basic business/promotional timing. But he was probably done or near done at least a month ago(not to say he didn't have bugs either way).
They also have revealed games/re-releases, so this could be a good time to reveal the Master Chief Collection for Switch. If not that, they'll just advertise Microsoft again.
Like, we have zero good precedent or information. At most the final characters were often niche. But that doesn't mean much. This is the second time for a Final DLC character, and there's an implication Byleth was meant for TGA but changed(likely to make sure their reveal and release were much closer to each other, something that's a good business move. Hype doesn't last long, so get individual sales as fast as possible).