Minor nitpick, but there's not necessarily a single super user in UNIX. You can create multiple users with uid=0 and they all will be super users.
The situation is the same for any userid and any groupid. If you try creating three ordinary users with a UID of 3005, they will be, essentially, the same user. There is no way at the system level to differentiate them, after they have authenticated. Because their files and processes are owned by the same UID.
This sharing of UIDs is generally discouraged and quite undesirable. It makes systems administration a real mess.
User is user. Uid is uid. It's not the same. Uid is used for file permissions, that's true.
You would be unable to enforce quotas or privacy for any of them. Whatever they did on the system would be indistinguishable, because their process UIDs would be identical. Any files they created would be owned the same. Sure, set them up with unique lists of GIDs; it really doesn't matter in the end.
I have no idea what you mean "User is user", but you are right: UID is not the same as a username. The username exists only in the passwd(5) database, and not in the kernel, like at all. The kernel has no idea what usernames are, and that's why they're irrelevant to user administration.