If you are fine with getting there 80% his claim is correct. That he's using the language for his games proves it to me. Of course, if you require it to be both open and popular to be a success you kind of reach a stalemate. In a world where everyone can quickly whip up a language I would expect most to be closed to the public anyway.
I would like you to elaborate on how still being in closed beta is an argument against the thesis. To me it just looks like avoiding feature-requests, nitpicks and community interactions for a product he doesn't even want to sell.
The interesting part to me was always that he didn't justify Jai very much. He didn't assert that anybody else would want to use it, or that it would be universally better than C++. His claim was that he didn't need to: building a language is now so quick that he can make a language just for him, just because he feels like it. Everybody can have their own language frontend.
It is pretty understandable he needs to hold back before he releases his new game, or even more.
Which means this website doesn't even know how HTTP 4xx codes work (406 is about content negotiation) and is also being rather pathetically pedantic about user agents.
So, to you polygonjs.com, I say, 469 Go Fuck Yourself.