Manipulation and decentralization can't be combined .

Manipulation can happen at any market. Whether it is stock market or crypto market. Manipulations by whales can happen. Stock market is centralized and yet it can still get manipulated.
Decentralization's definition can't be connected with manipulation at any case . Decentralization is something that doesn't have any central authority to govern something which in this case, no one is governing us so its safe to say that Bitcoin is decentralized in any way.

Overall, whales can manipulate any market although there are no concrete evidence that they are but it doesn't mean though that if a certain market is manipulated it isn't decentralized.