I have an old Sigg kettle from the early 70's that is thin spun aluminum. I have used it mostly on small stoves but a couple of fires also with no damage. I am guessing these boxes will have about the same thickness of Al and take the heat. Because of the shape, they may have lid fit problems, but like you say who cares.
I am setting up my Trusbx Picnic Box and Trangia box as day hike/car trunk/ACS emergency Ham radio station brew up kits. Small Sour Altoids tin stove, esbit pot support, alcohol bottle, and tea, instant soup etc. I would probably set up one of these Sigg boxes, due to the lid seal as a FAK, but would use to boil up water in an instant if needed.