why github claim it is distributed version control system ?

In centralized system like TFS, each developer is a node where he works on the code on the central hub. The code is same as the centralized code except the work you are doing. Before someone makes any changes to the code, he may have to take the same code as server (getting latest).

Whereas the GIT , gives you an ability to maintain the public repository that others can use to add as remote repository and make a change and push to either server or saving it as public repository allows others to do the same.

That’s why I used peer to peer word. This is just a brief of giant system but again, I would suggest to do some research or reading the official GIT workflow site for better understanding.