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.
A create post in a group, originates not from the group, but from one of its members, who may not even live on that server. Then that create needs to get pushed in some way to the federated community, which then needs announce to all its followers. Its very different from a person create, which always originates from the same server, and goes to its own followers.