Well let's look at Andross as another case. We know he isn't a character, because he didn't get a profile page along side his reveal, we already have seen him appear before as an Assist Trophy in Brawl, and We know he isn't apart of the Mario Galaxy games so he couldn't possibly be a stage hazard.
Already covered that. Much like Star Wolf, he could appear on any Space-like stage to do his move. I think he probably is an AT, but this possibility isn't that odd. He made Giga Bowser a Final Smash, so this is really about as weird.
Do you see how assuming he's anything but an Assist Trophy can come off as silly??? Idk, I'm probably alone on this.
You're not alone, but no, it's not silly either to wonder what his current role is. I haven't seen anyone say him being an AT isn't a good possibility, though.
I have yet to see why it couldn't be a Final Smash, I'm going to have to go back and re-read the last 2 to 3 pages.
But yeah, Sakurai's statement suggests that his Poltergust will be used someway in the game, coincidentally while he is also showing all the symptoms of having a final smash.
It could be his Down B. Before the Poltergust disappears, he activates his Final Smash, so it just happens to show up then and there. It could be a coincidence. He doesn't even say what the attack is, just suggests it might be this. Every other Final Smash actually shows the attack itself go off, or the full transformation where applicable... except this one. It's really suspicious right now that he barely made it look like anything was happening. This is why I'm taking it as a Final Smash with some skepticism. Because it's not as easy to say as the others. It has signs, but no attack, who says your temporary things(like FLUDD) wouldn't "stay up" till the Final Smash goes off? He didn't even activate it, from what it seems. Or he could've just pulled it out.
It could be a coincidence. That's all I'm saying.