Well, I'm not a customs agent, but I'd imagine they do it in the same way they adjudicate anything else: inspection. Some things get through by chance, of course, but not at a rate you'd want to rely on if you're a business.

In particular, if I walk into a random post office and send a one-off shipment internationally, the paperwork, origin, packaging, manifest, etc. is vastly different than what, say, Temu was doing to ship a $10 widget to US consumers at scale.

The rule you're talking about is not new, so presumably they've figured it out.