ActivityPub does not govern how user handles work. All AP actors are defined by their IDs (which in Lemmy's case happens to be the URL their profile is hosted in, which is a mistake as you cannot change your username without breaking federation, but at least Lemmy isn't alone, both Mastodon and I think *oma family of software do the same thing)

AFAIK the @username@instance convention is Webfinger's doing, and (to the best of my very incomplete knowledge) the convention of "preferredUsername @ the hostname of the object ID" is a hack Mastodon pulled that got adopted as a de-facto standard (as is quite a lot of other things in AP).