Couldn't one have a list of all instances and from that direct it to the app?
Like links to youtube get opened by the YT app?
Any lemmy.ml/... , lemmy.world/... , and so on get opened by the "lemmy-app" which is installed.
Jerboa does maintain a list of instances for you to select how to open in the app - list here and see my other comment on how to pick the instances you want.
The list can be updated with this generation script, it seems to get all the Lemmy instances with at least 50 users from fediverse.observer.