Stainless is not stain proof. Different grades of stainless have different levels of rust resistance depending mostly on chrome, nickel, molybdenum, and carbon levels, but even the most resistant grade will rust under some conditions. The primary method of rust protection, what makes stainless stainless, is a continuous layer of chrome oxide on the surface. The problem is that layer is weak to chlorides like salt or bleach so exposure to such can allow rust to begin. Adding more nickel, more chrome, and more moly can help but nothing is rust proof, even extreme marine grades like 317L can rust.

Edit: the cybertruck is made using their own “30X” grade. The most common is 304 or 18/8, which has around 18% chrome and 8% nickel. 302 has less, 309 has more. I would bet it is near 304 as that is the most common austenitic (nonmagnetic) stainless steel. This grade does not have the molybdenum added to “marine stainless” grades like 316 and 317. That molybdenum provides some resistance to chlorides which is why it is used in marine applications and commercial kitchens. It is also quite a bit more expensive.