Now, is F415 separated from F400?
It's not levels that are separated...or not, it's aircraft. A VectorLine has pointed out it's only a separation problem if there is another aircraft around.
Many years of working approach has highlighted similar concerns and misunderstandings around the Transition Altitude/Level. My job is to keep the aeroplanes separated by 1000ft vertically - I may temporarily assign an altitude to an aircraft above TL (or, heaven forbid, temporarily assign a level in the Transition Layer for 'cruising') in my known traffic environment, but so long as I achieve 1000ft vertically I've won the game. The rules do not stop me from doing such things. Whether it is good practice or a good idea is something else.