They are just federating, like the rest of us they aren't degrading the fediverse users of their current power and control. That is my counterpoint to your counter point.
Although I do agree with you regarding that you probably shouldn't trust them ultamately, but there is a different way to look at it considering the fediverse acts differently then the old centralised internet. and I think many users on Mastodon seem to not highlight that when they talk about Threads federating.