Actually, Canadian citizenship law is irrelevant to the discussion -- U.S. citizenship law is controlling. Understand that, quite logically, one country's citizenship laws cannot be secondary to another nation's citizenship laws.
Under U.S. law, Cruz was a U.S. citizen the moment he hit the delivery table. The only circumstance under which the U.S. would recognize Cruz' Canadian citizenship would be if he ever chose to opt for it (which he was eligible to do under Canadian law), renouncing his American citizenship in the process.
Since he never made that choice, Cruz was never anything but a U.S. citizen -- under U.S. citizenship law.
Your logic is misguided, and the facts are also against you