It really isn't true. It doesn't just work. I'm running Chrome 38 on a modern desktop PC and it doesn't render at a reasonable frame rate.
WebGL runs everywhere that is objectively a Good Thing™. Your criticism is aimed at the optimisation rather than the technology.
The person who created this could, theoretically, add a little slider to let you adjust the visual quality to meet your PC's performance characteristics.
Additionally WebGL can also be utilised for things which only get rendered once (e.g. single 3D frames). Like generating 3D graphs for one suitable example.
If they wanted to, they could even auto-scale the quality based on frame-rate to ensure that everybody had a good experience.
Right now, this is still maturing – e.g. for me that demo runs great even on an old iPhone 4S, 2010 MacBook Air, etc. but on a lowest-bidder Dell laptop with the cheapest possible Intel integrated graphics, it struggles. One hardware generation refresh, however, and even the cheap equipment is adequate for many uses - after all, that 2010 MBA was only $1k 4 years ago.
The upshot is that the excitement is because this is finally reaching the point where support will have matured by the time most people will ship a serious app. There are already some interesting WebGL-only applications like Polarr (https://www.polarr.co/editor) which suggest that the model is already plausible for many projects.