Although, calling it an invite is probably not good or clear communication on Discord’s part. If an invite is publicly posted for anyone to see and there’s no vetting of who is getting the invites… that’s just a public space.
Plus, even if the server is in Discovery (and thus really publicly advertised), they’re still mostly sorted into 10+ rooms. Just because a library is public, doesn’t mean I should expect Study Room B will be recorded by John Jameson and entered into a public dataset.