Them telling drivers to meticulously clean the vehicles doesn’t necessarily mean they’re afraid its going to rust. They’re probably concerned with staining and junk getting embedded into the panels. In the forum post, you can actually see a little bit of staining circling some of the debris on the panels. Furthermore, iron particles can get on the panel and rust themselves even though the actual panel itself isn’t rusting. This is why they make iron decontamination spray for auto detailing.