I'm not a 5 handicap, but I think it's a fair requirement in trying to build a pool of 2000+ raters. They need to have some proxy of general golf knowledge, and there probably isn't a better one than handicap.
Courses are also all rated from the back tees, by definition, and how difficult they are is a key metric; so having better golfers as raters is rational.
I just don't agree. Lots of 10 cappers are more in touch with golf than low cappers. Just because you have the skill to play golf doesn't mean you have the skill to evaluate courses.
And rating a course is 100% subjective, so why do you want only the opinion of low cappers? If 80% of the golfers are above a five, then why have the minority rate courses for the majority?
I can rate a course from the back tees just as well as a 5 capper. And why should a course rating be based on tees that 95% of us never play?
Maybe a better proxy is how many times you play per year.