That is different, though. The Vietnam draft dodgers committed crimes (dodging the draft) in the past. Their crimes were then pardoned. As long as they didn’t go forward and commit more crimes, they were set.
Illegal aliens committed crimes in the past, in the now, and in the future. As long as they aren’t supposed to be here, they commit crimes.
But only past crimes can be pardoned, There is no blanket pardon for the future.
What about diplomatic immunity? Maybe he declares all illegals are now diplomats...