He's mentioned it many times, but it's possible you're not a subscriber so I'll explain: Just because two players are in the same tier, it does not mean they are equivalent prospects in value or upside. He said there are obviously levels within the tier, where some prospects are near the top of that specific tier, and some are at the bottom of the tier. He's also mentioned that the bottom of one tier and top of another are not super far off.
A lot of people get caught up by headlines, as you can see fans of specific teams come in to complain about their rankings, but few people read the actual article to understand the context behind the rankings, which is really the value in Corey's work. I understand people not wanting to subscribe to The Athletic, but it's very clear that some are in here to complain without having read a word of the article, which is kind of pointless.