I cannot say it's usability break if using back button allows me to go to the previously seen slide and after that forward button brings me back where I was. The only problem is that you have to wait a bit until URL is changed. OTOH it is quite useful too, because quick scrolling over the slides isn't recorded in the history, only the slides where you were for more than a mere second or so.

Not to mention that it doesn't scale well with screen size (I can't see the bottom below tokenization on slide where it shows structure). It looks nifty, but that's about it.