This article is correct, and provides an example, in saying that the DOM can easily create thousands of elements in milliseconds. However, the problem is that events and interactions with it all happen on a single process. Things load and block the thread, an event happens and blocks the thread, etc.
The Sencha guys did something really smart. They used an object pool with one object: requestAnimationFrame, as the core of their mobile platform and sent EVERYTHING to it. That way events, loading, or whatever, didn't block, but happened in a FIFO manor. They also kept the number of dom elements static and just reused them when needed, so an object pool of dom nodes -- no creating or destroying nodes.
The things that make working with the dom slow isn't only creating nodes, but applying styles, listeners, and destroying them -- basically creating an application.
I do agree with the overall premise that a lot of developers do not know the best methods/practices/patterns to utilize when creating complex applications. I learn new things daily and I hope that our community continues to teach itself and provide tools to make getting things done easier.