I feel if we're getting a Gen 8 Pokemon, it's not so much promotional as a cool idea and it just took longer to get out than was ideal. Remember, the character was chosen quite a long time ago. Not that I think it's one of the more likely options.
But yeah, Byleth was done moreso to help with DLC than the base game of 3H. Sometimes characters are at best promoting games that no longer are getting new stuff and is just at best "hey, get the current stuff that's still out". This is somewhat the case for Incineroar and Greninja too, considering their timing doesn't really match up with promoting any content nearly coming out. Byleth is even closer than those.
On another note, it's weird the second tweet didn't mention Smash. Maybe they more meant to post that only and someone screwed up? Even if it's a screw up, it sure as hell is going to get more viewers, with what might as well be a guarantee for a Smash character.
(I also want to point out that in no way is Sakurai forced to add a Pokemon newcomer. He chose that, and Gamefreak noted he chooses the newcomers too. He might at best be tied to what characters he can add, but not the idea of adding one. There's little doubt that Gamefreak is practically setting up the plausible choices. At least the data we have only shows him selecting from a list. He likely had more freedom with some of Smash 64, since Jigglypuff's addition wasn't just cause a neat anime-related option, but it was an easy clone of Kirby. Mewtwo was the only other known one. And he might not have had much freedom regardless, since every single Pokemon character is already big in their own right. This is also why I can see ones like Urshifu being plausible, though not so much by this point, since that DLC is much more over than the current one. That is, a promotional Pokemon might be the best option Sakurai can go for. Albeit, in this specific context, Nintendo chose the character already, not him, so it could be more limited than that. But seeing as how they're all promotable Pokemon on their own merits, Nintendo might've gone for the same list as Sakurai would've had to work with).