I think a lot of people on HN would disagree with your assessment on email. Every other article is how "my startup sent an ad to everyone on HN and now nobody at the company can email Gmail users anymore". That's kind of an extension of RFC8222 that Google made themselves. It's good for end users, but it does make a lot of people mad. Back when there wasn't one central email provider, you could still reach users that had less aggressive spam filtering settings. If you made Hotmail mad, you could still reach Yahoo! With one provider, you screw up once, and you're dead. (As someone who once ran my own mail server on a consumer DSL line, I don't miss this world at all. For incoming mail, I eventually deployed the nuclear option of greylisting and man that broke a lot of stuff. For outgoing mail, all I ever did was send an email, watch it bounce, and then plead my case to the various denylist providers. It mostly all worked though!)
"You use Firefox? Oh. It worked in Chrome." is another similar thing; people have to emit Chrome's version of HTML or users won't be able to use their website. And then fixing your site for Firefox's quirks ends up being pushed down the priority list to zero. The Free browser ends up losing out to the Corporate one, even though they both ostensibly support the same HTML standards. That makes a lot of people feel icky.
It's reasonable to think that any sort of federated system that gets popular will have rules, restrictions, and quirks put in place that the original creators didn't really think about or care about. I think that's where the comment is coming from, but I also think it's inevitable either way. Right now, people say "Let's use ActivityPub because it's cool", but if it becomes too cool, then it will be "Let's use ActivityPub so that we can reach the 3 billion Facebook users". And then you'll only test your code against Facebook, and will be sad when they block you for spamming. Just like email, just like browsers. It's the way of the Universe.
But, that means the only alternative is "let's never become too cool" and that doesn't really work either. You can die in obscurity, or you can become beholden to a behemoth. I guess it's possible that decentralization eats the world and Facebook fades into irrelevance. It's already doing that anyway, as people discover "cooler" centralized platforms. To appeal to the masses, there will always have to be something that appeals to them and "I can self host it!" or "My provider can't be subpoenaed by the FBI" are probably not the sort of features that are making ordinary people beat a path to your door.