I would agree except that they also use scrolling to change slides. It feels a little weird to scroll down 3 ticks of the mousewheel, then have 3 clicks of the back button do the reverse action.
In the scrolling case, I still don't see how it's "overriding the browser buttons", but rather having a JavaScript that advances to the next page on scroll.
In the scrolling scenario, my actual back and forward browser buttons behaved as expected — just for the pages (slides) I visited. No more, no less.