He doesn't have it today. He had a similar right in Cuba as he did in the US. At the moment of his birth, neither country knew he existed, and that's how things would remain if he or his parents didn't take an affirmative step of claiming the foreign (to Canada) citizenship.
Cruz's parents apparently presented his claim for US citizenship to the US authorities, who adjudicated the claim in light of the evidence presented. This is what is done for all births abroad when there is a claim of citizenship under an Act of Congress.
Cuba, unlike Canada and the US, expressly rejects dual citizenship. Now, that does not mean that being born a Canadian, or having a right to US citizenship automatically disqualifies Cruz from asserting his claim to Cuban citizenship. In fact, he might, as far as I know, be able to assert it today. But he didn't assert it at what is considered to be an "appropriate" age, typically shortly after reaching the age of majority, and so, as far as Cuba is concerned, he is "unknown," his claim never having been presented.