Around here, "no rust" means the visible bodywork has no spots where rust has eaten through the paint. There is rust underneath the bonnet, under the car, on the brakes, etc.. but not on any normally visible painted surfaces.

The thing about snowbelt areas, is that corrosive salts are used as de-icing agents on the roads, and those same salts eat metal on cars. Thus the concern about rust.

Cheers