Again, what can they tap or see into that they couldn’t before? All info on the other servers is public, that would be true for any federated server. I really don’t get how they’d get any more access to your data than another random person on the internet seeing your profile. They’re not making their own instance available to make accounts on, or enable users to post on it directly. You aren’t giving them any more details than you would if you had a Twitter account that was public. It is quite literally just for official government information dissemination without being locked behind rate limits.

Surveillance? In what sense, here in particular. A bit confused. Also, it depends on the kind of private instance you mean, since this is private too, in the sense you cannot make accounts on it. What other benefit do they gain over people, using this over a corporate website?