I’m surprised existing systems weren’t mentioned.

Booting from the network has been a thing since the 80s and is now standardized, I guess the main ‘new’ thing here is using https instead of tftp

I didn’t see anything about port knocking there, it rather looks like it has the opposite focus - a quote from that page is “features that support widespread scanning of many machines are supported, while in-depth scanning of single machines aren’t.”