Yes a blended wing would be stiffer, but wing stiffness isn't really relevant here. The wing bending isn't evidence of differential pressure between the two wings, it's only showing that the wings are the part of the plane that's most affected by turbulence while not accounting for the majority of the inertia.

The point is that air currents powerful enough to appreciably affect a large plane are large scale and therefore you're unlikely to have enough differential pressure from one wing to the other to impart much torque.

I'm not certain about this and I'd love to be corrected if I'm wrong.