Can you explain how Google helped XMPP even in the slightest way? Because that's what I'm arguing against.

The only thing I can come up with is the increased popularity, which is shaky because tech-naive users wouldn't know or care about Google Talk's underlying protocol. Also, considering the rest of what Google did with XMPP, like making it hard for their servers to be interoperable with others, or their slow adoption of new features, it's clear to me that Google getting involved was a net negative for XMPP. I don't think I'm assuming anything to arrive on that conclusion.

It's true that instances don't need to grow exponentially (or at all), but most mods/admins want to maintain their community and not see it dwindle down to nothing. People used to interacting with instances run by Facebook or other corporations (which most of their friends or family will use) might get upset if the federation link with them gets severed. If they do, they'll either pressure the instance admin to comply with the corporations and federate with them again, or switch to the corporations' instances. Both of these scenarios are bad for the future of the fediverse.

If they don't give a shit about the fediverse why do they want to join it? Only Facebook can win from this.