One theory is that they don't buy it in a single move/day, instead they do it continuously at the lowest possible prices. So someone must have sold at those prices and because they bought at lower prices, the prices don't move up too much.

I don't think whales have been going custodial for their coins in a large number, may be a few who still believe in the system and think that it'd be a theft of tax or are afraid they could be caught under the law for selling at non-kyc, or are in need of huge fiat amounts.

TBH, I don't think you can rely on non-KYC exchanges when you need more fiat amounts or when an urgent need arises for a big amount.