I' m betting it was an M-14, they put a new barrel shroud on them so they look an awful lot like the old Garands at a glance. I think you saw this because most of the activity that is being taped is Marine action, and USMC still uses the M-14 as much if not more than the M-16 in a lot of instances.

If it is " without a doubt a carbine" , then it sure isn' t a Garand! DId it have a visible magazine below the belly? how about the flash-surpressor/compensator? I' m not for sure but if I recall the 30 Carbine M-1 saw less of a campaign than the M-1 Garand, so I' m sure it isn' t an M-1 Carbine.