Or the technology could just share the information it needs to share and not everything and anonymise the data ;)

It does. If it “anonymized” the data before broadcasting it to the federation, usernames would not be valid across federated instances.

If I post on instance A as “John” but my username gets anonymized as “UserA893SAJ”, any instance other than A has no idea that that is John, and therefore it is just some anonymous user.

It’s totally possible, but that’s not what Lemmy wants to be

Edit: Yeah no, in cases where attribution is not necessary, like upvote/downvote, they really should be anonymized between Lemmy instances.

I wonder why it isn’t at the moment. Possibly just didn’t have the foresight. I could look into contributing that possibly if someone isn’t working on it already