And there was of course, the first couple of presidents because they were technically born British citizens but still born in the colonies.
They actually had specific exemptions written into the Constitution for exactly that reason. Cruz has no such privilege, but he's still natural-born because at least one parent was a US citizen and we have citizenship pass by blood as well as by location. That he was also a Canadian citizen is largely irrelevant, since the Constitution doesn't blacklist non-American citizens, it whitelists American citizens.
Which raises the question of whether you're really a citizen of a place if your parents are, which gives you the right to claim citizenship, but you never do?
As far as I know, such an arrangement does not exist. You're either a citizen or you're not; if a nation gives you a right to citizenship contingent upon claiming it but you never claim it, you're simply never a citizen. Citizenship
can be revoked, for example at 18 if a nation requires you to choose to be solely a citizen of itself at that age or else renounce citizenship. However, as far as international law is concerned, you can stack citizenships to a very nearly arbitrary degree, at least in principle. It's just that each nation can impose arbitrary rules on how and to whom it grants citizenship.
So, to answer your question, if your parents are citizens of a nation that implements
jus sanguis, you are absolutely a citizen. If that nation, however, merely offers the opportunity for citizenship to all children of citizens, you are not a citizen until you complete the requirements for obtaining citizenship. This is a distinct situation from Kenya, which evidently totally allows dual citizenship up to 18.