It is absolutely a deportation threat. Clearly political, red meat to his base, on the eve of his 2020 re-election campaign. Like most threats, it's designed to create fear. And it's working.
1 - ICE was blindsided by the tweet. Yet another example of dysfunction.
2 - ICE doesn't have the resources to deport millions. (Being a thorn in his side is one of my life's great satisfactions.)
Now that he's said it, I suppose ICE has to do what it can to make it a reality. Or at least an illusion of reality.
1 - KNOWLEDGE IS POWER. The administration is counting on people running and hiding, not seeking advice, not standing together. There are thousands - millions - of people who stand with these unfairly targeted people.