If lemmy created a community the same name as the username when someone signs up. You could follow a lemmy user and then when your on mastodon (once we’re federated with mastodon) you could see their self posts in their community as a mastodon post and vice versa.
Yeah I understand that it’s a long ways before federation with other services happens. I was just curious if there was an ActivityPub software the devs would find better to federate with than Mastodon. Honestly I’m perfectly content with just Lemmy-Lemmy federation (: