The idiomatic way to query communities in Lemmy is to interrogate your instance, not to interrogate the instance hosting the community. I think there are some sensible reasons for this:

• Large instances hosting lots of communities want to delegate read workload to the many instances out there hosting users. They don't WANT everyone coming to query the communities instance directly. That's rather the whole point of federation. Now, will one app doing direct community-hosting queries bring down the threadiverse? No, it won't. But it's not how community discovery is envisioned to work, and Jerboa being developed by the lemmy devs means that it's unlikely to employ non-federated community discovery hacks.
• If you offer community discovery by directly querying the instance, you create another discovery problem which is equally hard to solve... which is instance discovery. OP may have a particular instance in mind already that they want to query, but as soon as querying communities by instance becomes a commonly used feature... people WILL immediately begin asking how to search instances to put into that list... which again is generally a problem that is supposed to be addressed through federation. Also if you don't build instance discovery, you'll have tons of reports from people who mistype instance names and can't figure out why it's not working.

All of which is to say that while there is an approach that involves directly querying a specific instance... it's a partial solution that doesn't build toward a comprehensive one. I don't expect the devs to move this direction, but rather to focus on fixing the community browser in lemmy and exposing those capabilities through jerboa. This is the larger job I was referring to, and although there is a shorter path to OP's specific request, I don't think it's a likely one to be followed in Jerboa.