There are reasons you might need (or want I suppose) globally unique IP addresses while not publishing said IP addresses to the global public internet. An operation on the scale of the US Department of Defense would easily find itself in such a situation. If you have private interconnections with multiple third party organizations (which may be other government agencies, even), private address space is not sufficient. Eventually you either run out of private address space or it becomes impossible to coordinate uniqueness among all interconnecting parties. Globally unique IP addresses as obtained through ARIN or its siblings or predecessors solves that problem. This is one of the ways you can trigger the "routing policy differs from upstream providers" thing in the allocation rules. So even if the DoD allocations didn't predate the current allocation rules, the lack of advertisement to the public internet doesn't mean they weren't using them under the definitions of current policy.
I can think of one good reason they might be announing the addresses now. There are idiot[1] network operators (of various sizes) who have chosen to use portions of the DoD allocations as "private" addresses on their networks. As soon as they do that, it means that anyone on a DoD network who has been assigned one of those addresses will have trouble accessing resources on other networks who have used DoD space as private addresses. By announcing the space, they are firing a shot across the bow of any organization dumb[1] enough to be using the DoD address space within their network.
Of course, they may also simply be "cleaning" the addresses in preparation for transferring to other entities (government or otherwise) or they may be planning to use the addresses publicly or some combination of both. Announcing the addresses allows identifying a lot of the problems.
[1] Yes, I do mean idiot and dumb.