I think it's fair to make the guess that insiders may have completely wrong info but to argue that these insiders and journos don't want to "upset Nintendo" or whatever is one hell of a tinfoil theory. It makes it sound like Nintendo employs some of the more staunch "deniers" of this leak as some sort of personal fall guy, and that's a bit of a humiliating assertion to make that I highly doubt any self-respecting journalist would consent to. If any of these guys blindly get these things wrong, that's a pretty big blow to their journalistic integrity to be honest.
Consider how almost none of the names we've seen speak up about the leak have actually had that much of a history "leaking" Smash info, it doesn't really behoove any of these guys to talk about this particular leak either - and it's not like Gamexplain has been roadblocked from covering this particular rumor either, they already have two videos about it on their channel. The only thing they haven't done is a discussion about it - and the easy answer to why they haven't is because it's probably really hard for them to casually discuss why they don't believe in this leak without giving away knowledge they're seemingly privy to. A lot of people who fall into Team Real like to make a case for themselves that people are overthinking the leak in regards to what makes it "fake", but that's somewhat ironic given the conclusion you'd have to reach to assume that GX or anyone are willingly coming out against this just because Nintendo will get mad if they don't.
The easy answer is a lot of thirsty fans are getting their hopes up, and people who've been privy to knowledge in the past are suggesting not to do that because what they know outright contradicts it. If they're wrong about that, that's fair, but there's no conspiracy at play here.