Further, github isn't looking to "extinguish" git in any way. Sure, old workflows (emailing patches?) might not be supported, but again, that's something entirely external to git itself.
As these things are all external to git, it makes sense that they're not portable between vendors- they are the vendor's, not git's, features. You're not using them from within git, you're using the vendor's features to interact with git. That's the primary difference between, say, M$FT's EEE of Java or AOL's instant messenger, HTML and other examples.
Even if everyone used github, they're still using vanilla git underneath.
The same is true for tracking forks and managing pull requests.
Attached to the platform, but open. You could do all the work on your feature branch somewhere else, say, your company's gitlab installation, then when it's done, push the branch into your github "fork" and and create a pull request there. I really don't know on which side of an openness line that would fall, you can easily define the line around it either way.
You’d have to run half a dozen commands, including adding new remote and running “hub fork”, but it would likely still be faster than sending an email.
The important question would be if Github forks and pull requests are an open protocol or attached to the platform. I’m not aware if I can make a pull request from - say - bitbucket to github. Can I ? Then it’s not a power grab. If not then redefinition of fork/pull request is an extend and extinguish move.
Edit: okay, extinguish is too harsh. GitHub doesn’t want to extinguish git.