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.
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.