Is starting with the technology aspect really the right way to go?
It depends.
If you have a pretty clear vision of the overall goals, without having every detail written down and defined, it makes sense to start creating the technology first.
The best idea is worthless if you cannot make it happen. And no 200-page-design document will change that.
So if you know you're going to create a city builder, it makes sense to build a technology first that can handle many buildings, lots of calculations going on in the background, etc.
During this step, you may notice that you will not be able to deliver everything, so you saved yourself the trouble of writing down all those little details that you couldn't implement anyway.
Once you have your basic technology running, you can start defining all the detailed game mechanics and implement those. That way, you will be much faster implementing, testing, changing, etc. a single feature. NOW is a good time to write your 200-page game design document and create all the tasks to prevent feature creep.
But I'm not a big fan of defining every little detail first and then implementing it, anyway. Game design is a highly iterative and feedback-driven process. You will never know if your idea really works until you see it in action.
About this actual project...
I'm
very skeptical about the used technology here.
JavaScript is just slow as hell compared to other languages like C++, C# or even Java (stuff like asm.js and emscripten helps, but let's be honest, it is a looooong way till performance heaven). Though I can very much understand the decision, as that is what the developer is comfortable with.
Also, the game will basically be its own browser, so the problem of normal browser games - you have to share memory and cpu/gpu power with all those other open tabs - is at least reduced.
Still, I'll have to see it in action to really believe that this works. There is a reason we do not yet have highly demanding games running in the browser written in JavaScript. And I'd say a halfway modern city simulation is rather demanding.
