I agree, but it is curious that the KDP recommendation is to provide flattened CMYK with no profiles (PDF/X-1a with no output intent!).
It is, for whatever reasons, probably what works best for them—in that, taking in mostly amateur/novice files, it give a more consistent result than the whopping spread that RGB might bring.
As for 'recommendation,' if you've spent any time reading or trying to keep up with KDP's rules, regs and standards, you know that they're about one-third verities, one-third out of date and one-third too vague or novice-speak to be useful.
Don't get me wrong: I think KDP, all financial and corporate-political issues aside, is a huge boon for small and even not so small publishers. But while print quality is good and trending better, color management is never going to be better than the local FedEx Office or Kinko's.