[Raynor]

I would also have problems with the origin of the quote "mightiest of the descendants of Thorondor". It might just be a conjecture by hobbits getting mesmerized by newfound information and legends from both Rivendell or Gondor. Or even more, who in Middle-Earth could claim knowledge about events of the First Age?

I think we can also dwell a lot on the meaning of this kinship between Thorondor and Gwaihir & Landroval. It might be just a kinship of legacy, affiliation to the good side, a similarity in standing among their kin, might and abilities they possess. All in all, this phrase seems like an interjection from someone whose identity completely unknown to us (Bilbo? Frodo? some scribe?) with no backing from anyone else at the time of the events - not that we can expect many, if anyone, to be able to testify on this issue, to begin with.