Sure, if they actually have any. You’d need a way to verify the code was actually written by them. You’d also need to be careful you’re not overly biasing towards “people who [have time to / are allowed to] publish on GitHub”. Plenty of good developers don’t/can’t use GitHub.

You don't really need to test basic skills over and over again.. you could just ask them what part of the code they wrote, if they don't have any open source code then ask them to send you a take-home code assignment they completed for another job interview, or have them complete one you make for them.

You're right that there are plenty of developers that have no experience with GitHub, but I'm sure there are far more who would struggle with simple theoretical questions and programming quizzes during the interview, because it's difficult to think clearly when you're feeling a lot of pressure.