There were no flat panels. That was a sales pitch. In reality, a slight curve to a panel adds so much strength that it can be made considerably lighter/cheaper than a flat panel of sufficient strength. Flat panels also tend to dent inwards very easily, which is not a good look for a car.

I had a jeep wrangler once. The window was "flat" but not if you looked at its reflection. There was a slight complex curve for strength.