He's talking about applying the change retroactively, which isn't normally how things are done. There is however some historical precedent. The first part of the 14th amendment was specifically written in response to a supreme court decision in 1857 that ruled that black African Americans and their descendants were not US citizens and therefore not protected by the constitution even though they were born in the US.

He's arguing that this could be used as a wedge to retroactively revoke citizenship, and frankly with the monsters currently in the SCOTUS it's a not entirely unreasonable fear. I could easily see the current crop of Judges ruling by a slim majority that in order for a child to have citizenship they must prove not just that their parents had citizenship, but that they must prove their parents had citizenship not through birthright, which would effectively require finding a descendant that either had naturalization papers or else could trace their lineage back to the revolutionary war. Ironically this would make it easier for immigrants to prove citizenship than it would most "Americans". For this reason if nothing else I don't think this is a particularly likely outcome.

It's probably worth while to look at what this would mean in practice though. The motivation for birthright citizenship was specifically to protect the children of slaves and their descendants who were brought to this country against their will (and arguably should have been considered citizens at the point they entered the country, but that's a whole can of worms I don't want to open right now). Their children who were born here had no choice in being here, but spent their entire lives here and I can think of no good argument for why they shouldn't be considered citizens. Likewise in the case of illegal immigrants, while they chose to be here, their children born here would have no say in being here or not and once again I can see no reasonable argument why someone who was born here and lived their entire life here, shouldn't be considered a citizen.

Imagine for a moment that we didn't have birthright citizenship. You're an illegal immigrant, you're pregnant and about to give birth. What do you do? If you go to a hospital, you'll have your child, but without immigration papers the child won't be a citizen. How would that play out? I think we can all imagine a world in which ICE would be on hand to "arrest" the newborn and the mother and start deportation proceedings. So, as an illegal immigrant you would decide not to give birth in hospitals, but instead at home. Now you've got generational non-citizens. So, you're the child of an illegal immigrant, born and raised in the US. You're pregnant, and now you're faced with the same decision, maybe you give birth at home, maybe you go to a hospital. In either case at some point one of these descendants is going to get caught by ICE, but now we've got a problem. They were born in the US. Maybe even their parents were born in the US. Where exactly do you "deport" them to? They have no citizenship literally anywhere in the world. Do you just go off their name and features and take a wild guess? You think Mexico or China or whoever is going to just accept their new "citizen" just because the US doesn't want to deal with them? How is this even remotely moral either? You're taking someone who lived their entire life here, where this is all they've ever known, and you're going to send them to some random country they've never been to, and probably don't even speak the language of?