If I understand correctly, users of other fediverse things like Mastodon can follow lemmy communities (and users?) but the reverse is not currently possible. Would it not make sense for lemmy users to be able to subscribe to a hashtag or user on a mastodon instance?
The extra comment in that FAQ
seems to indicate that it is indeed not an actual follow in the AP sense. Note however that it does bring into my timeline posts from people which I don’t follow and which are on other instances.
So in Lemmy terms it seems that it is like searching the “All” timeline for the hashtag and including the resulting posts in the “subscribed” one. But of course the feature asked about by OP would involve that operation on a bigger “All” timeline that also includes toots.