What they are saying is that there is a player ranked 30th (last place) in the Ultimate Arena and also happens to be ranked 5th in realm arena. So no one can attack that player in the Ultimate Arena because no one can see him in there due to be 30th. No one can attack him in realm arena because he is ranked 5th there and 11th place can only see up to 6th (Ranks 1-10 can only see the Ultimate arena).

No one can see him in either arena, therefore he cannot be attacked in the Ultimate Arena and cannot be moved out of the Ultimate Arena. This effectively makes only 29 slots available for players in the Ultimate Arena. It has nothing to do with how strong the inactive player is.

Hope this clarifies it for those he don't seem to get what this thread is about.