I mean there are ways around that (i.e. activitypub proxy) however I think the key point to remember is that by defederating you are saying no and not giving consent. If somebody decides to spin up an instance of activitypub proxy to go around that, it's a very different situation than choosing to federate.
That's the important key when you don't give them consent and they barge in anyway it changes the dynamic, and makes them look bad, to all but the most brainwashed chuds. However when you let them in that won't happen, it's just on you (you being instance owner who chose to federate).
So the "we should federate and give them consent because they're going to take the data anyway" argument is a really bad argument because it devilifies them in that situation, because they were given consent to do it by federating, but weren't given consent when they set up instances of activitypub proxy to skirt the block.
I know the majority of people making this argument aren't even going to read this response, and I'm sure that a good amount of them are just astroturfers (there's already threads astroturfing elsewhere online, it wouldn't be surprising to me to find it in the biggest Lemmy instance out of all of them).