The bot gets its data from https://lemmyverse.net/ - which isn’t aware of https://walledgarden.xyz/. I don’t know why, but there’s a problem with the site that might be related - if you do curl -i --header 'accept: application/activity+json' --location https://walledgarden.xyz/
it errors with ‘transferred a partial file’ whereas it works for any other Lemmy server. It must have worked in the past though, 'cos I was able to subscribe and post to a community on there not long ago.
!lemmydirectory@lemmy.dbzer0.com seems interesting, and it only took me 4 months to find out about it. Dunno if the list of communities it covers is being updated anymore though.