Now if Canada attempted to force him into its army, he might be able to claim exemption on the basis that he was also an American citizen. But whether that would be accepted or not depends on Canadian law, not American law.
Canada could force him into their army if they so chose. They could not force me into their army. There is no recognized body of law which would allow them to do it. Ted Cruz? He owes them allegiance.
Didn’t read the law, did you?
5.2: A person who is a Canadian citizen under paragraph 1(b) ceases to be a Canadian citizen upon the expiration of three years after the day on which he attains the age of twenty-one years unless he:
Either is living in Canada or files a declaration of intention to retain Canadian citizenship.
Since it is likely neither applies to Cruz, he has no citizenship status with Canada.
This law is from 1946, and it is at least possible the law has been changed since.
BTW, this issue of dual citizenship applied to Winston Churchill, who was a dual citizen of the US and UK till his 21st birthday.