React's rendering works differently than traditional JS apps. He's not re-inventing the wheel, he's adapting it to a new vehicle.

Also, hell, if you want to make a good demo project then re-writing something is a great place to start. A demo project that got me hired was just a designer and I's answer to Adobe Kuler.

Using all of ExtJS just for a grid seems like a bit much. It's no surprise that when a new framework comes about there's a landrush to reinvent all of the common components.

I build a bunch of big reports with ExtJS. Its grid-component is just superb. Infinite scrolling just blows up the browser after a while, but not with the ExtJS grid.