I see it as a just-in-case message, to let you know that you’re browsing a non-kbin community and that some federation features may break on either side, even if they are not broken at the moment. There may also be some differences in how posts and comments are sorted between kbin and the original instance, and - I’m not sure if that’s exactly how it works - potentially you may see different comments depending on who the two instances federate with.

Thus far he’s proven to be a great guy, but in case he lives long enough to become a villain, yes - he owns this instance, and if he’s ever taken over by some space parasite, he has the ability to shut down this instance and delete everything uploaded to it; but the beauty of the fediverse is, if you don’t trust an admin of this instance, you can join a different one, and still talk to people here; if you don’t trust any admin, you can spin up your own instance, and it will still be able to communicate with all the others; and if you don’t trust anyone, even yourself… well, fediverse will not solve that problem.