It is possible, but highly unlikely. Center hydraulic systems controls normal gear extention/retraction. Can 'free-fall' gear as long as you retain some main battery power. The fact that the tail-skid was shown extended tells me the handle was in the down position (it goes up and down with the gear handle). It will be interesting to hear what issues this aircraft had. Don't believe that has ever been a 767 gear up landing in its 25+ year history.