AT's are heavily decided as very specific gameplay roles as characters who couldn't make it in as playable.
Spirits were mostly ideas by his co-workers to replace Trophies while having a gameplay purpose.
They're not comparable at all. They're in very different positions.
So no, those are not convincing reasons. Even then, he absolutely could make Waluigi playable if he wanted to. He would do it due to a lot of fan demand. It would sell too. That's more than enough. However, since Nintendo is choosing the DLC, unless it came up, he would have no reason to pick it. Unlike Spirits who have tiny roles, any of them could easily be playable characters while not affecting anything else. He isn't going to go out of his way to say no because they're a spirit. That's pretty nonsexnsical at this point. They're nothing more than trophies that are more than "nice to look at", but barely much more. You'd have a remote point of them mattering if we didn't have multiple Mario spirits as well. If it was that important, we'd solely have two spirits, both fighter ones, for Mario and Dr. Mario and that's literally it. Guess what's not the case, completely eliminating the issue? Bandanna Dee, for instance, could easily be a DLC character, and all they'd have to do is rename the in-game wording for Dee's actual spirit. That was easy. That's the extent of work he has to do.
Why would he choose any spirit? Why wouldn't he. It hurts nothing and takes no real work to fix the text display by any means. It only makes more fans happy to see their cool character in. He at most approved of each spirit, something he noted recently. He is not going to keep a list of around 1300 spirits for the sole purpose of "lol, these guys cannot be playable". That's assuming he's super petty over nothing. He isn't. Have a big more faith in the guy.