I'm not convinced this is a good argument. You're certainly over-stating how teachable the GRE is, and I have a least anecdotal evidence of lots of people who scored above 90% on the general GRE quantitative section without tutors. This includes at least one person who "took it cold." Maybe those are super exceptional folks, but I think the statement that most of the people scoring in the top 30% had tutors is a really strong statement. I have worked for a test prep agency before and there aren't a lot of top tier students in those classes, and indeed the courses and techniques really geared towards the bottom/middle-tier students. Also, you can't do well on the GRE, especially the subject tests, without knowing the subject matter.

Your argument is plausible, but it's all conjecture. I'm curious as to whether you think the GRE percentages mean anything at all, and if so, what the 'adjustment' for taking it cold should be,