How is that utterly useless? It's pretty clear the intention is to prevent a) people being prevented from leaving enmasse and b) revoking their passport.

This is basic stuff and doesn't work in a vacuum you can safely assume we live on the same planet that happens to have mostly open borders.

Regarding the customary international law thing it was not refuted at all. It says that it does not constitute international law for the USA, and does not constitute domestic law for some [citation needed] other countries - likely because of the vague at times wording.

The fact some nations dispute it doesn't make it any less customary international law, hell it's kind of the definition otherwise it would just be international law.

https://en.m.wikipedia.org/wiki/Customary_international_law

Customary is just that, it doesn't have to be upheld but it can be used as a moral guide to defining and shaping other laws.