lorenfb said:
Yes, there's a small pin hole (air port) in the top of the cover. If the vehicle is fully submersed, yes water will eventually enter the shock tower.
The 1mm hole at the top is probably not the problem,
it's the seal around the edge. And if water from the windshield does for some reason get routed to dump directly on top of the cover, like from driving winds or poor drainage design, then a 1mm hole is big enough to be a problem. A small amount of water that takes 14 days to evaporate can be worse than lots of water that evaporates in 2 days. It's only the water molecules that touch the metal that cause rust, not the rest of the water floating on top of that layer.
But if someone already has the covers installed, it's an easy test: after driving home from work in the rain, or after going through a car wash, open a cover and stick in some tissue paper to see how much water is picked up. Or if someone has had the covers installed for a few years now, what does it look like underneath? No need to speculate.