I can see my lemmy.ml uer from mastodon and even communities (called groups on there).
From the smell of it it seems something that mastodon allows, for specific federated services, but it’s not out of the box for all activitypub fediversed services/instances
Edit: what i find strange is that there is a clear way to verify websites to me, with a rel=me relationship. But there is no clear way to say “those other federated identities are the same of me”.
I get that the rel=me way is well known and well used, but allowing for this concept in the protocol of a federated service seems to be important.
At least, I care about the concept of digital identity and I would think for a distributed and federated and ever evolving network like the fediverse, this would be quite a common place to be