And look at how Adblock has gone

I think you missed the point here. Forking is and always has been a totally viable hedge against any other parties control in an Open Source product. Google can't force Microsoft to take it as it is with no input because Microsoft can absolutely fork. Just like Apple and Google forked from each other.

The real difficulty is that you need someone with large pockets to fund any forks if those forks are going to be viable. And that is due to the complexity of the web as a platform.

The person they're replying to straight up claimed "Google doesn't have control over Chromium", which to me reads most naturally as treating the unforked code base as a community project where anyone can submit commits.

As you noted, I don't think forking and maintaining a Google sized code base is a realistic alternative. But by the same token, I don't think that the possibility of forking said code base is what people typically mean by not having control.

> Just like Apple and Google forked from each other.

“Each other”? Google forked from Apple; Apple forked from KDE, not Google.