this seems more like a Mastodon thing, considering that if I curl the post with Accept: application/activity+json it doesn't have a link to the Lemmy thread, it has the link actually embedded in the post

I think Mastodon does this for other AP object types (like articles, e.g. if you put a link to a WriteFreely article into Mastodon it'll just show the title and a link to the original post)

@self