The bit about diesel residue is a common fear, but diesel dries up really quick. A good wash with soap and water would take care of any leftover stuff. But yeah, using fuel to clean cups is never a smart move.
My uncle's garage always had this weird mix of smells, coffee and motor oil. It's a specific kind of carelessness, using whatever's close at hand without a second thought. I see it when someone cleans a paintbrush in the kitchen sink or wipes a greasy tool with a good towel. It's like the brain just shortcuts from 'this is dirty' to 'make it clean' without the middle step of 'use the right thing'.