P2P is a form of decentralization. Your resource doesn't back up your claim.

E.g. it states: "A collection of decentralized computers systems are components of a larger computer network, held together by local stations of equal importance and capability. These systems are capable of running independently of each other." This is true for Matrix with Homeservers being the decentralized computers and the network of the same being the larger computer network.

"Decentralization" referring to P2P is exactly what I said...?

If you are going to pick a tiny part of the page, you might find that it's not sufficient. In the sense of your chosen quote, Twitter is decentralized, since it runs on multiple computers organized in a network. Those are capable of running independently of each other, to a degree, if Twitter Inc did their High Availability work correctly.

It is not decentralized from a user's perspective though, since the software they use on their devices cannot run independently of the remote systems of twitter.com.

I don't know where that leaves us, but then again I don't know what you were trying to argue either.

Just because P2P is a form of decentralization, it does not need to be the only form of decentralization. Federation is imho another form of decentralization.

Yeah, and I agree there are different types of decentralization: e.g. physical, authority, ... or combinations

Each incarnation with different goals

Sure, this is not the only possible way those words could be used, which is why I mentioned a "consensus". Many words can be used in ways that are "technically correct" but confusing. Like I said, by some definition Twitter is "decentralized", however it is not what anyone ever means by "decentralized".