Well, I’m entirely willing to believe the US DoD is one of the few entities that have more than 2^23 computers they want to be mutually addressable, so the RFC 1918 space is just too small for them if they are to run IPv4.
In fact - DoD should just have it's own internet - that is completely separate. I'd argue that DoD networks should not have any connectivity with broader internet, making their use of the whole 32 bit space completely independent from everyone else.
Doesn't mean that DoD cannot use the global internet, just not use it and their own at the same time. I mean... They could even send traffic over global lines, without taking up IP addresses.