They don’t falsely advertise themselves as decentralized. They are decentralized even if you’ve come up with your own definition where there can be multiple “centralized” entities in control of the system.
There is no central authority in mastodon. There are many entities that are part of a federated system, just like email (which is also decentralized).
Nostr is also decentralized but it’s decentralized by a relay (? – the name of this sort of thing isn’t super well established; they’ve never really caught on) system (which is a twist on peer-to-peer models that overcomes some of the issues with peer to peer tech) instead of using federation.