[Strange Raven]

Or it more simply says that Trolls are not very bright, and really don't know or care if there is a difference between a king and a steward.

Then again, Denethor was a megalomaniac and whose to say Ecthelion was not the same and may well have styled themselves as kings. It runs in the family.

Or, they could be referring to an otherwise unreferenced Troll King.

Any of those things could explain away a minor discrepancy that comes of writing the books separately.