As someone who regularly writes both JS and TS, I'm fairly certain that this dichotomy is not real. There are places where TS makes more sense (eg large, trustless applications with many devs) and places where JS makes more sense (eg libraries that would otherwise need to have very abstract types).

I'd understand opting for either JS or TS as a policy going forward, but it's hilarious that this dude probably worked himself up so intensely about this that he brazenly rug-pulled his own repo and invalidated all open PRs.