I asked this question ages ago and it was pointed out that "sub" isn't a reddit specific term. It's been short for "subforum" since the first BBSes, so it's basically a ubiquitous internet term.

"Sub" works because everybody already knows what you mean and it's the word you intuitively reach for.

You can call them "communities" if you want, but it's longer and can't easily be shortened.

I just call them subs now.

Lemmy is a selfhosted, federated social link aggregation and discussion forum. It consists of many different communities which are focused on different topics. Users can post text, links or images and discuss it with others. Voting helps to bring the most interesting items to the top. There are strong moderation tools to keep out spam and trolls. All this is completely free and open, not controlled by any company. This means that there is no advertising, tracking, or secret algorithms.

Lemmy Community

Sublemmy is cringe and doesn't work very well as a portmanteau

Maybe there's some word theory out there to describe why it doesn't work but I don't know the name of it

I just call them communities. That's what I've seen others use.