The US doesn't keep biometric data on every citizen or lawful resident, and the government can trivially lie or make a mistake about whether they did the database/ID check. This isn't a hypothetical, many legal residents and some citizens have been swept up, and without due process they have no ability to say, "I'm a legal resident" or "I'm a US citizen". They can just be shipped off to an El Salvadoran prison camp where the president can claim, "oops, I can't get them back because they're not in our jurisdiction any more".