Anyone with a copy of the repo can set up a mirror, so although they are using GitHub's centralized publishing platform, they are not dependent on it.
Git is one of the most widely used distributed protocols on the internet. GitHub is just one node on the graph... seems like a decent use/definition to me.
Distributed is not the same as decentralized. GitHub is centralized and distributed.
I'm not sure if GitHub fits the definition of distributed (unless you're talking about their load balancing setup, in which case - is that relevant?), but git is designed to be a decentralized store of information (specifically code, but not stricty) and optionally distributed.
Git, the protocol, is not the same as GitHub, the service. Git, the protocol, is absolutely distributed. And just to repeat my point - GitHub is only a single possible node in the graph