You can view them, but you can’t interact with them. When I try to upvote a post or comment when lemmy.world is selected I get an error message along the lines of “This thread was retrieved via lemmy.world. You are not logged in.” Forgetting and then having to search the post on your own instance to interact with it can be a bit annoying.

Edit: I just saw that they added an option to switch instances when viewing a community in the latest version. Great news!