The royal example you state is a bastardization of the Constitutional intent. If either William or Kate WAS an American citizen, the child WOULD be American, and eligible for the Presidency. Bob
if kate were a US citizen, the child would still be a british citizen via william. yes, he’d be a US citizen as well... but not naturally as he can choose either.
same with bh0bama, mrubio, bjindahl, and tcruz
most Americans are unfamiliar with this as they were born on the soil to 2 US citizens. myself, i was born on US soil to a US father and a scottish mother. just like bh0bama, i am a US citizen as well as british (it’s not uncommon for people like me to have multiple passports)