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.
Even so, you shouldn't underestimate people's ability to have a meltdown during the pressure of an interview.. I won't comment on the React question, since I've never used it and don't know what a 'reducer' is.. but even the truly bizarre for-loop you posted is likely to cause confusion to some people during the interview. If I saw it while I felt the pressure of an interview, then I'd possibly feel confused and would need a couple of minutes to understand what was happening. This would no doubt reflect poorly on me, which is unfortunate because when I don't feel the pressure of the interview then I can instantly understand (and laugh at) the code.
I believe you'd have much better results if you ascertained the applicants technical ability before the interview, and instead spent the interview getting to know the person, to see if (s)he had a positive and friendly personality. If none of your applicants have side-projects or code on GitHub then perhaps you're searching the wrong places, and/or failing to properly promote the position (especially if some of your developers are earning 2xxK). I'm sure if you posted the position in HN's 'Who's Hiring' and included a 100+k salary, then you'd get flooded by talented developers that have open source code/impressive side-projects. And if somebody with an impressive resume doesn't have that, then you can just request a small take-home assignment that's related to the work you do. If you'd like to avoid getting flooded with resumes', then you could also just reach out to the developers you find interesting on GitHub.. considering your salary range then I imagine you'd have a very high interest rate.
Judging by your posts in this thread, have you considered the problem might be that you (and many other companies) hire people based on their ability to answer theoretical questions (which also happens to be while under a lot of pressure), rather than their ability to code? I'd like to think there are plenty of candidates that have open source projects that showcase their coding skills, or that have made impressive side-projects.
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.