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

Short answer is “no.”

Slightly longer answer is: “all of your public posts on Lemmy or Mastodon or any other federated platform are the Public web. So no, it’s not different.”

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.