that is not entirely true from a lemmy perspective. When an instance subscribes to a community, the remote instance gets the last 20 or so posts, as well as subscribes to all new posts from then on. IT has a local copy of that community. What it doesn’t have is any of the embedded media.
AFAIK, this is similar to how matrix works too. I do not know if this is a lemmy implementation choice, or a AP standard?
Edit: haha, i just saw that i am the 3rd person to say the same thing. oops!