Extinguish doesn't mean to end, it means to end the open standard. Github would be happy if all git use were Github use, and every user locked in to their service brings that day closer.
In a similar way you could argue that git doesn’t change diff and patch. It just adds a a convenient way to handle patch files. So why open source it? It turns out it’s very useful to have it in software.
The same is true for tracking forks and managing pull requests.
Even if everyone used github, they're still using vanilla git underneath.