How do you know rulers of China, Russia and USA aren’t best friends ever and share information at top levels? You simply cannot trust a third party vendor anymore, no matter where they reside and this includes your cpu too.

Any information that needs security from nation state level requires physical data diodes.

I guess you can use the method you mentioned for protection against lower level actors.

What does this mean in practice? Can you walk us through how to secure email using such a process?

What he's describing only works for symmetric encryption. Assymetric is the problematic one. There is no real way to verify the trustworthiness of VPN/proxy systems.