This works because the Group is public, making the content within it public, regardless of the user's personal privacy settings.
If you can find their Instagram handle (often linked in their Facebook "About" section or searchable by name), their Instagram might be public. You can view their stories and photos there without sending a friend request on Facebook. how to view private facebook posts without being friend
When a Facebook user sets a post to “Friends only” or a more restrictive custom setting, Facebook’s systems enforce that access control. There is no “backdoor” for non-friends to see that content without the user’s permission. This works because the Group is public, making