If they were born here then they are citizens and nobody is advocating stripping them of citizenship.

Not if the next president gets away with it: https://www.latimes.com/politics/story/2024-12-16/trump-said...

It's unlikely he'll succeed, and he walked back on many campaign promises.

You are confusing what he is calling for. He is not advocating for people who are already citizens to lose their citizenship. He is saying, going forward, people who are born here will not automatically be given citizenship.