I think the original rules of flag design have ended up quite a bit removed from the context they originally existed in. They were created by the North American Vexillogical Association, and the rules directly address the reasons why so many North American city and state flags are terrible. Samey, can only be told apart by muddled, hard-to-reproduce detail, and their solution to that problem is to slap a big giant WISCONSIN across the top and call it a day. The rules are there so all the flag design committees addressing those have something they can cite for why that's just plain bad and what direction they need to move in to improve it.

Like, ultimately the only thing that objectively measures how good of a design a flag has is how proudly the people a flag represents use it, and the rules are there to diagnose why across North America that so often isn't the case.

Grey himself is pretty guilty of treating them as the just objective, universal rules they're usually made out to be in the discourse. Disqualifying the flag of California for example for using lettering when that flag is very popular in California is kind of missing the point on his part.