Science is the belief in the ignorance of experts.
I also looked it up on the middle earth map and noticed that the Ninglor is nowhere near the Carrock so that art wouldn't make sense.
My understanding is that Ninglor is the Elvish name for the Gladden River which is about 150 miles south of the Carrock. Given the scarcity of named locations around the Carrock, having a Ninglor location seems reasonable to me. Sure, Rivendell is about the same distance away but it is over the Misty Mountains while the Ninglor can be reached by boat down the Anduin.
Edit: Elven -> Elvish