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.

What exactly do you think they'll be able to do now?

They can see pretty much all the things without an instance. So can you. Social media is not private.