A person can renounce a citizenship. In fact, it’s not uncommon when people apply for jobs requiring a security clearance. Then, a person may renounce, say, his English citizenship, and be a US citizen only. It happens all the time. Then, you’re no longer a dual citizen. I don’t, however, think that’s exactly germane to this discussion.
If we follow the Cruz NBC definition we defy the purpose of the clause.
I could donate sperm to a woman in Turkey, who would give birth to an NBC. That child could then pass NBC to his children, in Turkey or Saudi Arabia, or where ever he chooses.
There was a reason for the clause. Electing Cruz violates the purpose, on its face.