This is not correct - Tyvek is not completely waterproof - only highly water resistant, at least according to DuPont.

Silnylon will not 'leak' under high pressure. It will 'mist' very gently under extremely high hydrostatic pressure. However, most shelters made out of silnylon are shaped to resist any sort of pooling of water and reflect such pressure. In my experience I have only had misting one time, and it was ever so slight, during a 40 mph rain / wind storm. I was unable to use any natural wind breaks in the area.