Hm... that may or may not be accurate. Proof needed (maybe Dr.Proof has proof), and or one would surmise that one would be given the hat along with the title...
It does make sense though... He may had died before he was acknowledged. Personally I believe he died before getting a shot at wearing said hat in question, as he may had died at the same time, as they were pretty inseparable--even in death. See Edo Tensai.
As we can clearly see, his "title" is on his back; something too represent he's a Kage; unlike Tobirama if he were too go out on a mission, etc. Some uneducated ninja may mistake him for a non-Kage, is what I mean. Only at a glance though. His skills would of course make them think twice. Still, that said, the hat /title is of some importance, as they would not even wear them in the first, methinks.