Maybe you right. But! But if we want to know that — we must make same scene in 3d and watch fps. Unlikely my way to create terrains very easy to use. Even with shadows. And at my desctop it gives me an impressive 900fps (thanks to 336 shader processors).
In other sight — quad is 2.5D engine from time I start to develop it. There is a lot of 3D engines. There is a lot of 2D engines. But I need very impressive 2D image for my games. I like 2D.
This is not "just a demo".... heeey, look how I can!!! NO This technique was developed for my game. And game can be look like this:

Notice that FPS is real and all gfx drawing with pretty small amount of code.