I just reread the 4 books and I didnt see him claim to be a dragonborn. Only 2 spots even talk about dragonborns and thats one where the author calls him him Dragon and King. The second only talks about 6000 years of hidden bloodlines that came to Arena which we got a game explaining that situation.
I understand and I believe myself that the amulet of kings are won by kings and have actually nothing to do with being dragonborn.
MK wrote it with dragonborns in mind in wearing it but was changed in oblivion that theres a possibility of kings wearing it and not dragonborn. The mythic dawn guy can be explained since he was a king to the mythic dawn cult. Im fine with it, but people say its retconning to fit the storyline. Or could be seen that way as since there wasnt any books or transcripts that pointed that way to explain the possibility of kings wearing it and every transcript stating only dragonborns can wear it. Retconning or bad story telling, just depends if changing original view on the subject to fit story as retconning.
http://www.uesp.net/wiki/Oblivion:Mythic_Dawn_Commentaries
offering myself to that daybreak allowed the girdle of grace to contain me. When my voice returned, it spoke with another tongue. After three nights I could speak fire.
Mankar Cameron offered himself to a beam of light, which taught him to speak in another language, and after three nights he could speak fire.
Does that sound like anyhting to you?
1. When you kill a dragon, or get knowledge from the Greybeards, you get surrounded by a beam of light
2. Dragon's speak in a diffrent language
3. After THREE nights he could speek fire, what comes in threes? Shout words, specifically Yol, Tor, Shul or fire breath
Not to mention he wrose the Amulet of Kings, which is something only Dragonborn can do.