Nah, nearly 90% of mobile users interact with Reddit via the official app (most people use Reddit on mobile devices).

They can also get that data without doing anything because any data they’d get from federating is already public.

You can choose to show “Subscribed” communities—only the ones you’ve chosen, “Local” communities—only the ones on Lemmy.World, or “All”—which will pull from all the communities federated with Lemmy.World.

So to answer the question, posts from outside your home instance will show up in your feed, should you choose for them to.