ACW, that is probably the true story as with a pair of F4/F111/F15D there would need to be 4 stupid ones not two.
However 1989 seems a little late so maybe he used DUMMY not to confuse with an earlier use of STUPID
It was not an impossible trick though. The original chinagraph was waterproof wax. The later version was water soluble. Also quite a few aircrew adopted the finer felt tip which was also water soluble. Use one of those on you knee pad on a wet day
I am sure many of us have been there with vital notes/frequencies smudged out.