Cruz had as much right to Cuban citizenship at the moment of his birth, as he had to US citizenship. Cuban law attaches citizenship to a child born abroad to one citizen parent.
“Cruz was Canadian at birth, and would have been so for his entire life, regardless of where in the world he lived, except for affirmatively renouncing his Canadian citizenship.”
Incorrect. Each country has its own laws.
Clue #1.
So if the Ambassador from Russia is flying over Japan on a airplane owned and flagged by Singapore on their way to Canada, and Mrs. Ambassador gives birth in U.S. airspace and then the plane lands in Canada, and the first place the baby touches land is Alaska
in which country does the child qualify as a citizen?
Potentially all of them.
The fact that under the laws of various countries, one might be a citizen of several countries is irrelevant.