you guys are idiots.
this guy is working the exact correct range for back.
the full hang only means that you engage minor muscle at the start, its a pointless waste of energy. Furthermore this guy doesnt give his back any rest at all- its in constant tension the whole time- making it even harder. The body of the muscle is used from about 1/3 up to 4/5 up.
its the same as for bench, biceps curls and a whole load of other movements.
the form is fine.