If it behaves like an empire, it is one. I already pointed out that the British Empire had a period when it was not led by a monarch.
The difference is legitimism. The monarch leads because that is his right. Interregnum periods do not negate that principle. Instead, in leftwings quasi empires like Stalin’s the dictator dictates because he can. In EU, there is a democratic pretense of legitimacy. It is truly a “third way”.