[i-LINK]

Not sure if it's "impossible to believe," but I suppose everyone else is lying too.

I searched through dtic.mil w/ Google and there's a great amount of reports that detail DoD-funded research and projects that involved Common Lisp. This one is the most unambiguous:

https://apps.dtic.mil/sti/tr/pdf/ADA303486.pdf "The Composite Warfare Model (CWM), a Navy simulation used successfully by the Space and Naval Warfare Systems Command (SPAWAR) and others, uses a fairly radical decomposition in which most attributes and methods are in their own class. Objects may have hundreds of super- classes. Written in Common Lisp, CWM is "linguistically advantaged" for this approach; ..."

An amount of those could've never been used (some are just proposals, and I'm not reading more or searching to see if they went anywhere) but some of them certainly were. It's mostly simulation and planning software, perhaps not mission-critical (I'd think garbage collection would be an obstacle there).