Your link isn’t working for me.
I’m guessing they weren’t allowing them circa 1970, the year of Cruz Jr’s birth, to maintain Cuban citizenship.
Nevertheless, the general rule under the 1947 law was that they could NOT have dual citizenship, so that a strong indication that Cruz Sr would have dropped his Cuban citizenship when adopting Canada, and there’s circumstantial evidence that Cruz Senior did not continue with his Cuban citizenship, i.e., that he renounced Canadian citizenship when he acquired US citizenship.
There is no evidence that Cruz took advantage of any exception.