The question is: Are the grandchildren of foreigners also foreigners if one or both parents were born on U.S. soil? What about naturalized citizens?
Being born on US soil does not make one a natural born citizen, the last King of Thailand and Boris Johnson were born here.
Once a person has renounced previous citizenship and taken the oath to be a US citizen they are no longer a foreigner.
Once they are citizens their children are natural born citizens, if both parents are citizens.
If one is a foreigner, the child inherits that foreign nationality.
That same concept is how Ted Cruz had any claim to US citizenship.