@rapha3l I'm not really sure, to be honest, but I got a feeling that it's sort of both.
What I know for a fact, is that it's the same as if you entered the remote post's URL into your Mastodon instance's search bar. But I'm not 100% sure what doing this does to federation.
AFAIK, 'federation' isn't a binary matter. I.e. it doesn't appear to be a question of two instances exchanging everything or nothing.
As far as I can tell, federation (at least in the context of Mastodon) is based on who follows whom and what. That's why, for example, even though I follow several people on mastodon.social, I still need to fetch many replies from there.
So, I think that's what's happening here: The two instances will be federated in that they are aware of one another, but they won’t be exchanging every message going forward.
That is my understanding anyway. I hope someone will correct me if I'm wrong.