Bad Science
"The flexing and bending of a wing in flight is intended by design, and a very rigid wing would break much more easily. Skyscrapers are also designed this way, to actually sway a little – it makes them far more robust."
Nonsense.
They could make the wing more rigid, but it would weigh more, reducing payload, range, and efficiency. They don't actually care how much it bends as long as it doesn't break.
For civil engineers, it is cost vs bending, though the deflection limits are normally set by Building Standards/Codes.