“She was born in US. She is an American citizen.”
The Constitution draws a distinction between citizen and natural born citizen. Natural born was a legal term that existed at that time, and was made a requirement to be president. And when I had American history in the 70s in college, it still was a concept.
The problem for you is that the law, including the Constitution, does not distinguish in its text between "natural born citizen" and "citizen by reason of birth". You can posit that such a distinction existed in the minds of the Framers, but that's all it is until it's established in law.
The law only knows "natural born citizens" and "naturalized citizens". Harris is certainly not a naturalized citizen, so she is a natural born citizen, according to the law as it exists now.
But she should renounce her Jamaican dual citizenship if, God forbid, she is elected.