Any ground water will have more minerals than wain water. For example, the water delivered to my house has 450-550 mg/l TDS. A quick search shows that is about 10x higher than rain water [1].
My kitchen sink also doesn't get salted during the winter, the roads do. I wonder if someone at Tesla forgot that some areas rely heavily on salt to keep the roads ice free during the winter.
It's probably not rainwater that's causing the rust. It's the water from the pavement, which does have higher mineral content, and might have corrosive substances, such as salt.
[1] https://www.chaitanyaproducts.com/blog/rainwater-healthy-com...