If he was a natural born American citizen, how was he born into Canadian citizenship?
The same way Trump was born into British nationality. It's a matter of the laws of the foreign jurisdiction, and nothing to do with our laws and practice.
At least Cruz has renounced his Canadian nationality. When did Trump renounce his British nationality?
Trumpets are just so stupid!
Being a natural born citizen and being a citizen of another country are not mutually exclusive. If two married. Americans work at the US embassy in London, England and have a baby in London, the child is a natural born American citizen. At the he same time the child is a British citizen cause he was born in London. Get it?
He was born with dual citizenship - Canadian because he was born in Calgary, American because his mother was a citizen. That is not uncommon when children of US citizens are born in countries with birthright citizenship.