I would've sympathized with the notion of dismissing insiders for granted if this was just a matter of Vergeben speaking out against it (who is pretty snide, not gonna lie) but a lot of information he gets enters the grapevine before he gets a hold of it, and since most of the people who are speaking out against this leak have spent most of Ultimate's lead-up being fairly quiet they don't really stand anything to gain from coming out and saying "eh, this doesn't seem right" in regards to the veracity of this whole shebang. If it was clout they were chasing, they would've come out and leaked the roster beforehand, but that's clearly not it. Besides; this roster as is proposed reads as the biggest fanboy dream ever, and many of these people with inside scoops are genuine Smash fans, so there's not really much in the way of ulterior motive to want to diffuse this if it weren't for the mere idea that this entire leak, from what we know, is a very unrealistic proposition.
If anything I feel it should be taken as a tacit admission that if this stuff doesn't turn out real you'll only have yourself to blame for overhyping it. The Smash fan base is one of the most dedicated bases online but it's simultaneously often incredibly irrational and vindictive. At this point the leak has spread so hard that a lot of people are undoubtedly going to be angry when this turns out not to be real, and I feel that's going to end up doing a disservice to the hard work and effort Sakurai and his team has poured into Ultimate.