Legally? Only in that this legal matter is given to the BATFE to interpret, and they have to enforce it whichever way seems to fit the wording of the law best.
I can't tell you that a rip-cord mechanism IS a machine gun, but it seems a lot closer than a hand-crank mechanism. With a hand-crank, you can crank one turn (or half-a-turn) and get one shot. Crank two turns, get two shots. (Or whatever the rate is.) With rip-cord it would be, pull the cord, get a bunch of shots. One operation, multiple firings.
It would be so for no other reason than how these rip-cords operate, generally. Every "rip-cord" device like that I've seen you're spinning up a heavy flywheel that drives a reduction geartrain which operates the device. So if you hooked that to a trigger mechanism you'd be pulling the cord, spinning the flywheel, and then it would run the gun continually until that flywheel ran out of momentum.