Either this is not yet baked enough to explain or the authors were aiming for humor.
> Sketch can be implemented in any programming language...
... as long as it is react.js?
> Sketch can be implemented in any programming language...
... as long as it is react.js?
That said, I feel like this is learning something like CoffeeScript. You have to learn all this language specific syntax (though simplified), why not just learn the language itself at that point?
That's why I just maybe it still needs more time before seeking wider feedback: It's not just one example in react.js, it's basically all of the supporting arguments being based on react, with all of the examples in react.js.
Maybe they have a point, but right now the only support they for this proposal is react.js. A few examples showing the output in a few languages would help, which is why I say this needs more time to bake.