Yes, if you perform mmaps calculations asynchroniously... well, what happens to the creature that is supposed to be walking on the mmaps path? Does it just stand around for a while until it's path finished generating? Not to mention its AI code depends on knowing if a path is possible before it knows how to react to players, and mmaps is such a small amount of calculations that it wouldn't be worth the context switches needed to run it separately. It makes more sense if you just wait until the calculation is done before proceeding.
There are instances where more threads would make more sense, like running map updates on separate threads, but this isn't one of them.