What makes bitcoin decentralized?
I think you're not understanding what decentralized exactly means.

Sure, a company with a large stake can control the price ( up to a certain degree), but this does not mean that they control (the protocol) Bitcoin itself.

There's no central power which decides what transaction is good, or which one is bad, or who controls what.

which would still be the case even if company X had 90% of the bitcoin supply.