At a technical level, the Mozilla homeserver will be as decentralised as any other server in Matrix: i.e. entirely independent, and federating with the rest of the Matrix as normal.

At a legal level, it's true that the server will be admined by New Vector via Modular.im - but this is basically Mozilla just outsourcing their sysadmin to Modular, while also helping fund Matrix development (given that Modular subscriptions are used to fund development on Synapse and Matrix in general).