That’s not entirely true, there is however a fairly specific set of actions you need to do to renounce your citizenship. If you remember the drama around Isis women wanting to return to their countries of origin the legal question a lot of times was whether they had effectively renounced citizenship, not that they had joined a state that America and such didn’t recognize as existing.

The answer was mostly no, btw, they hadn’t actually renounced American citizenship in the way required, not that actual Constitutional definitions or legality matter to the state when it considers you an enemy, which is something these chuckleheads never really seem to understand.