I don't think a github profile (or any specific activity therein) should be a prerequisite for being considered a 'serious developer'.

I am of the same idea. Requiring online, public repositories, specifically in GitHub, doesn't cover all developers. It covers many of them nowadays, so there's that.

Yeah, it strikes me as a very... Silicon-Valley-brained view of software development.

For example, I'm sure there's a scary-talented engineer working on fluid simulation algorithms for a major aerospace manufacturer, and they would be fired if their work-product ever left the corporate VPN. Meanwhile, their personal stuff is in a self-hosted solution.