Yeah, https://lemmy.ml/communities/listing_type/All/page/1 is populated by communities that:

• Exist on an instance that lemmy.ml federates with (the federation list for lemmyl is at https://lemmy.ml/instances if you want to see what servers are on the list) AND a user has interacted with the community, usually by searching with the bang or url syntax from the communities page.
• OR if it’s a local community. Then it shows up automatically on creation.

So the way to join is to…

1. Visit https://pathfinder.social/communities, right-click on a community you want to join and copy it’s url.
2. Then go to https://lemmy.ml/communities/ and search that url. Weirdly nothing gets returned, I don’t know why. But you DO need to conduct this seemingly broken search (or someone does, once anyone searches, the community gets added to the list).
3. Then search for some keyword in the name of the community (like “pathfinder” or “off topic”), and NOW the community from pathfinder social will show up.

I know this procedure is feckin weird and makes no sense, but you need that first url search that returns nothing to prime things… then the second search will start to work and you can click the join button to subscribe. Like I said, it’s pretty rare to have to do this on lemmy.ml since it’s a big instance and people have already searched for most communities and so they’re in your list. But as a very new and pretty niche set of communities, you may be the first subscriber to these communities on pathfinder.social and so have to go through the searching dance.