This doesn’t explain things well. Lemmy is like a bunch of Reddits communicating with each other. The graphic makes it seem like there is just one Lemmy. Also, are Lemmy posts on Mastodon? Mastodon was largely empty last I checked.
Edit: I just now noticed the second Lemmy
I think the ATProtocol is better when it comes to connecting between different social media types. But I think ActivityPub makes better use of different servers.
Though I think something like Lemmy is difficult for both because of how different it is from most other social media types.