I think I may be too late for the OP, but just found this thread.
We have only been to Hawaii once, but we stayed at the POLY that same summer.
I thought it felt authentic, but we experienced Hawaii a little differently than some. We stayed with family who were living there at the time. So we did not stay in a resort or fancy hotel, so can't really compare that.
We did visit the Polynesian Cultural Center, which was very similar in style to the Polynesian Resort. But I don't know how authentic the cultural center is, though.
I do know a WDW fan who was born and raised in Hawaii, with family still there. She said the Poly felt authentic to her, and it is her favorite WDW resort.