Birtherism took out Cruz. Maybe that’s why Trump brought it up...but that would be another conspiracy theory :)
Birtherism is an Alinsky tactic to discredit anyone who wants the Constitution honored by excluding those from the Presidency those who are not natural born citizens.
One can only be NATURALLY a US citizen when one cannot possibly be anything else.
Cruz is doubly ineligible due to his Canadian citizenship and his foreign national father. It is unclear what citizenship his father held at the time of his birth, but he did not become a US citizen until 2005.