Bitcoin is completely decentralized, one or more individuals, institutions, and groups do not control Bitcoin. Bitcoin whales are organizations, individuals, and groups that hold 1000 or 1000+ bitcoins. Yes it may be somewhat true that bitcoin whales may temporarily have some effect on the bitcoin market. For example, if Michael Sheller sells all his bitcoins in the market at once, the bitcoin market will be affected to some extent. Bitcoin whales will not completely destroy the Bitcoin market.
It's decentralized as a network. Unfortunately, in terms of owners and price movers it's becoming more centralized.
As Bitcoin managed to prove its worthiness as a commodity, big investment companies and banks are trying hard to control it and throw the majority of small investors out of the market. Therefore we are watching a lot of FUD and manipulations lately. It won't be easy for the crowd to keep on their share of the market, if the real plan for Bitcoin is to be a global asset.