That does clarify the claim being made. So the force being returned by the frame represents an additional load that your foot has to push against (torque applied to pull the chain forward + torque applied to oppose the flex returned to the drivetrain). Sounds like more work, not less.

Edit: to make my point abundantly clear, "the flex returned to the drivetrain" is weasel wording. The flex, a.k.a. force, that is returned to the drivetrain doesn't magically leap through the bottom bracket and chainring and into the chain.

If what's happening is anything at all like the description says, the returned force is pushing the pedal against the foot, presenting another load that your foot has to work against, in addition to the load represented by pulling of the chain to drive the wheel.