Their JS in not minified, you may want to take a look.
To accommodate for small screens/windows it scrolls down until it hits the bottom and then goes right. On the way back it scrolls up first, until it reaches the top, then towards the left.
Now middle-drag works (press and hold down mousewheel, drag for direction of scroll).
Also the scrollwheel on my mouse happily scrolled the page to the right when I pushed it to the right.
When I looked at it the first time, the mouse problem didn't came up. I used my two fingers and like in the authors mind I just swiped to the left and the timeline moved as expected.
After reading the comments, and seeing the 'mouse problem' I could see it in my minds eye how it 'breaks'.
But seeing your comment I realised the web site only breaks for 'older' input devices.
I believe the website would work just as nicely on any multi touch cellphone, and it would make sense to scroll from left to right.
and iPhones and iPads and android phones and kindle tablets and ....
Web traffic for some (most?) sites is already tipping over the half-mobile-usage direction (where "mobile" means "doesn't have a built in full-sized keyboard").