-> That other "anachronism" thread
I'd like to shamelessly reiterate my musings made there that such possible anachronisms can just be translations, and that the cosmology of Middle-Earth need not be consistent with what is traditioned history from our point of view.
Also, I'd like to point to statements such as in the foreword to HoME VI which relativate the importance of The Hobbit and the early LotR chapters for the (canon of the) Legendarium.