An hour record really isn't a great example of something that calls for stiffness. Aero is all. Of course, aero was somewhat of a fringe concept when Merckx did his ride.

As long as the chain stays on at first, it really doesn't matter how stiff/not stiff those bikes are. I always thought it was funny that Merckx' bike was lightened to an incredible degree for the time. Just not that important once it gets up on plane. Which is after the first 15 seconds.