As I understood it, it isn't a matter of pulling power (if it's that, then two side by side fixed lengths would do the trick) but the length of travel.
The bungee cord has a limited extension ability which is too short to fit the body motion's range (if it's tied between the knee ascender and the bridge). That's a concern from the beginning. Haas is one clever of the solutions found.
By putting the low anchor point for the bungee very low, near the foot, you get almost twice the useful length (but the same strength). Even more with the small block added at the bottom in the new Haas.
Problem, you're fine with the bungee and it's working range, but it no longer pulls the knee ascender, since the ascender is above the lower anchor point. If the ascender can't be pulled, makes it be pushed up by a sturdy tether from the foot. The bungee only limits the sideway movements and avoids the thing to flip upside down. That's not a strong control (side load on a line) but that's enough to work. The tube inserted in the tether gives the sturdy property and at the same time a guide for the bungee.