I understand the concept, but I think that they've violated the user's expectation. On most sites, tabs do not support the back button, so why should they be different? The back button is what most users will click on when there is no other obvious way to undo their previous action, but in this case there is. As such, I think the expectation of the back button is to go to the previous site, not the previous video.

Hopefully, however, they did user studies and found that I'm wrong. I, obviously, have not done any user studies, so it's easy to believe that would be the case.