Can't be blanks. You need to back pressure of the bullet leaving the chamber in order to cycle the weapon. In training exercises, the military uses something called a blank firing adapter, or BFA, which looks like a muzzle brake and is quite obvious to notice.
*disclaimer - I haven't watched the video, as it is blocked, but from the thumbnail it sure doesn't look like it has a BFA attached. So unless he is cycling the weapon manually, there is no way he is shooting with blanks.
You're assuming it was a normal rifle, I'm assuming it was a replica, like the ones they use on tv, designed to only fire blanks.