Whenever I find a «brand» new way to work with CSS (or whatever they wanted to call it), I'm skeptical. Most of the times they just redo what IDs and classes were made for, but way worse. While this might have good intentions in mind, it's just adding classes via attributes.
Also it's explained very badly, I had to click three buttons in order to get examples.
Agree with the rest of the stuff you've said though. I just don't see a place for this library.
Also I am fairly certain (been a while since I spent time in Webkit source) but class name and attribute selectors are implemented the same, if you want to do a proper benchmark you would want to do id, nested selectors, attributes, class, etc. And at that level you will find a significant difference.
Setting some Booleans as attributes is much nicer. However, it is built into CSS already, so not sure a library would help...