> There's Supreme Court precedent establishing that this isn't the case.

This? https://en.wikipedia.org/wiki/United_States_v._Martinez-Fuer... or is there newer now?

> ACLU itself had backed off it, last I checked

They did, the current page greatly narrows the scope of their border-zone guidance to the SCOTUS case I linked before: https://www.aclu.org/know-your-rights/border-zone