The constitution is not a suicide pact. If deporting millions of people without due process is authoritarianism, what is letting in millions of people without due process? Rules for thee but not for me?
Nobody is saying this should happen; they are saying that if someone is accused of being in the country illegally then we should investigate that accusation and attempt to arrive at a conclusion that is as close as possible to the truth. Indeed, treating that as optional is authoritarian.