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!