The policy says that ICE field offices are not subject to a federal law that allows members of Congress to make unannounced oversight visits to immigration facilities that “detain or otherwise house aliens.”
In short, ICE policy is to illegally use the threat of shooting to keep elected Democrats from being able to make inspections they're legally allowed to make
Once again, people can say "this is illegal" all they want.
But nobody seems to be willing or able to actually do anything about it. Chuck Schumer can call it a "silly piece of paper" all he wants, but right now he and his colleagues are being held off by that silly piece of paper.
Is anyone going to do anything about it? No. Is anyone going to force their way into these facilities? No. Is anyone in ICE going to face consequences for spinning this policy out of whole cloth? No. Which means, for all intents and purposes, what they're doing is de-facto legal.