Citizenship of the Union shall be additional to national citizenship and shall not replace it.
I see that you are now using the tight definition of citizen, but there are others, equally valid: senior citizens; citizens of a town or city; a citizen of a particular culture or social group; citizens of a geographical area such as Africa or Micronesia. Being a citizen does not always equate with being a 'national' of X.
What you cannot be is a citizen of a place that does not exist, such as the State of the EU.
It is true that I cannot provide a fully accredited legal document that the EU is not a state, that is because none need to exist. It is also true that you cannot provide a fully accredited legal document that the EU is a state. Opinions can be had from both sides, but as you pointed out with the A50 discussion they are only opinions until such time as they become facts.
The EU might have enhanced observer status in the UN, but the Arab League and the Red Cross are other organisations with normal observer status. None of them have voting rights nor the right to sit on the Security Council. Their status does not confer statehood.