It's pure fearmongering, US cannot quit NATO because it's a pillar of US imperialism. The only purpose is to blackmail EU governments to pour more money and to deepthroat the boot farther.
Realistically though, what would change in practical terms if US quit NATO. Europe made itself entirely dependent on US at this point, and they're terrified of Russia. So, they're still going to be buying weapons from US, and they're still going to be economically subjugated by US because they don't have other options. Seems like leaving NATO would just give US more flexibility to focus on Asia while making Europe more desperate which keeps it firmly under US control.
Imagine the US pushing Europe into war with Russia and then at the last second is like nah, you got this, keep buying our overpriced hardware and you'll be fine.
I wouldn't be surprised if the blob is on board with this given that Vicky just got dumped. Ukraine is going to end in disaster for NATO, and US sees China as the main rival.
So, it makes sense to pivot to Asia and leave Europe holding the bag with Russia. Military industry will still be selling weapons to Europe, but US will no longer have any actual commitments.
EU already put itself in a position where they're entirely dependent on US militarily and economically with or without NATO.
Do they really think Trump would quit NATO? I get that even if there is a 5% chance of it actually happening, they might see it as good to prepare. IDK if a Trump presidency would be good for the world. I imagine he'd continue the farce in Ukraine and also protect corporations from accountability like any other bourgeoisie president would do. He'll also persecute trans people and ethnic minorities to distract from the deteriorating conditions of capitalism
I get the impression that Trump faction is isolationist. So, while it will absolutely be terrible for people living in US, it might weaken the hegemony further.