This feels like the same argument:

"Al Gore claims he's an environmentalist, yet he flies using his personal gas guzzling plane."

And yeah, it's an ad hominem. "Is a fallacious argumentative strategy whereby genuine discussion of the topic at hand is avoided by instead attacking the character, motive, or other attribute of the person making the argument"

You can most definitely use centralized servers to disseminate decentralization. I'm pretty sure https://ipfs.io started as a github project and on a monolithic server backed by cloudflare. That doesn't dismiss them in the least - considering they now heavily dogfood.