I’ve talked and corresponded with Michael a lot over the last 17 years (not regularly during all that time, but pretty frequently during 2017–2020), and I don’t recall him ever saying anything about Mage: The Ascension. That doesn’t necessarily mean he’s never played—I never asked him about it—but it seems like some amount of absence of evidence undermining the “gets a lot of his material” claim.
I would strongly guess that you’re contributing to the phenomenon where gossip networks just make things up.
That’s not actually responding to the criticism. Rat culture could just be wrong!