The EU does have its own territory and population. Being supplimentary or not, that may be your definition but I'd argue your are finding arbitrary items to make a point. The constitution countries of the UK have effective supplementary citizenship. The same is true of sovereignty.
Now if that is your definition, it is true that the EU is not a country and neither is England. But it's very arbirtary as if it was written into the constitution that England could leave the union whenever it wanted, the UK would not be a country... Which is clearly nonsense.
Again if England had sovreignty and let the UK government by consent.. the UK would not be a country. This is clearly nonsense
It's not my definition, it's the legal attributes of a nation state. I'm not daft enough to make that up.
The point about supplementary citizenship is the art 20 of the TEC and the point about the attributes is every constitutional courses you will find.