I've been held ~24 hours by DHS under RAS, it is definitely a thing near the border where they don't need PC to jail you. They put me in jail but I was never under arrest, I later got my federal arrest record and there was nothing. Maybe that was what they were referring to?
Jailing citizens with no warrant nor PC was happening to me under Biden so it's not new either.
Correct you can be detained based on reasonable suspicion. You cannot be arrested on it. The difference might seem subtle, but 1) it's not, and 2) it certainly shouldn't be to actual DHS officials. It's literally in the text of the Constitution.
And yes, near the border has had shaky Constitutional protections for a long time. To suggest it's "not new" to expand that shakiness across the entire nation, eliminate time constraints on it, then scale up the "immigration" enforcement arm 100x is just laughable though. Of course it's new! It's new in scope, scale, and intensity. That makes it new!