I feel like the author and/or the team is confused about what HTML5 actually is. They're probably right to conclude that JavaScript/HTML5 won't perform as well on lower end devices, but a few things struck me as odd.
They mention Facebook switching away from HTML5, but I think Facebook is using the actual DOM, whereas their game would be using the canvas part of the HTML5 spec.
Later they mention WebAssembly replacing HTML5, but what it's really replacing is JavaScript, and the actual canvas that you're rendering to will stay the same afaik. The impact on performance would probably be as they predict, but overall, their issues seem to be with JavaScript, not HTML5.
> but what it's really replacing is JavaScript
That is a common misconception, but there no evidence of this. WASM replaces Flash and Flash never replaced JavaScript.
WASM replaces Javascript as the target of other language compilers. If you already use Javascript, WASM won't be much help. But WASM lets you compile C/C++ and with a bit more effort other languages into a bytecode instead of weird looking Javascript, or its more streamlined and still weird form, asm.js.