"...that USAID's mandate is not post-conflict (or nearly post-conflict) zones, but humanitarian aid."
Certainly didn't jibe with my recollection of what I'd seen them doing in a number of countries over the years. I knew that they'd been chopped to State a few years ago -- a bad mistake, I thought -- so I went to their web site and checked. They still have all their old missions. (LINK) and I'd be willing to bet they're in the priority listed:

* economic growth, agriculture and trade;
* global health; and,
* democracy, conflict prevention and humanitarian assistance.

Nation building and post conflict work are related and every one of those tasks is a fit.

That's not to say that some in the agency would rather not do that mission... .