I'd like to provide two pieces of potential evidence for ATs not deconfirming:
Spring Man was in Min Min's trailer, they did try to convince you he could be in the game
Sakurai never said anything about Spring Man's AT being the reason he wasn't chosen, just that "everyone's the protagonist!" and that ARMS' director wanted her in the game
This is the basis of the argument I put forth on my twitter.
https://twitter.com/SsbuWaluigi/status/1275084929819447297
I was initially very pessimistic because I figured this reveal would just be a Min Min showcase without much allusion to, or regard of Spring Man. I figured the worst case, that Spring Man would just merely be sidestepped because he is already an AT. I am very happy to be wrong about that. Not only was Spring Man acknowledged, but Sakurai himself even gave an apologetic as to
why he was not chosen. And in doing so doesn't even offer (in our perspective) the most powerful reason: he is already an Assist Trophy. I could very well have imagined that if this rule was that legitimate to Nintendo/Sakurai as well he could have said something like:
"When deciding the best candidate from ARMS, we chose Min Min(because Mr. Yabuki +etc). Some of you may be thinking Spring Man is the protagonist of the series, but this doesn't seem to be the case.
We also felt that given Spring Man has already entered the fray as an Assist Trophy, it would be a great opportunity to represent another popular character from the series."
I know it's a hypothetical scenario, but I really could see them make a statement like that if that was a major reason they did not choose Spring Man. And we all can see that he didn't make any statement like that. Basically, it seems obvious that they didn't choose Spring Man because he is already an AT, but Sakurai did not encourage that notion in his reasoning. And this might seem like a stretch, but when Sakurai said, "To be honest, it's because Mr. Yabuki wanted Min Min", it's like he's saying that's the main reason she was chosen out of all of them.
And like you said, the fact that Nintendo kept reinforcing the notion that Spring Man was a viable candidate....seems to literally imply Assist Trophies are viable candidates on its face. You can say something like this is a special circumstance that won't happen again...but yet and still the fact remains. An Assist Trophy being considered for a playable fighter was directly encouraged by Nintendo.
We also saw two things of note from the showcase:
1) The fan-made spirit rule is debunked.
2) Spring Man will also now appear in Min Min's Final Smash despite being an AT.
So not only are spirits eligible(another fan-made rule debased), but Assist Trophies can also appear in Final Smashes. This means at the very least Assist Trophies are not bound solely to their AT status and can be represented in other means. Is playable character excluded? Maybe.
In my opinion, the best argument against all of this is to say that Sakurai/Nintendo had a secret reason behind not choosing Spring Man other than the ones he gave. Openly he says one thing, but privately they mostly did it because Spring Man was already an Assist Trophy. Personally I don't believe that though.