I believe that while only half-literate, the Nord author of that book knew a lot more about what he was talking about than the author of "The Alduin/Akatosh Dichotomy", who's argument was "Alduin must be Akatosh because that's what we in the Imperial City have always believed, despite having no direct access to any descriptions of Alduin." His disregard for oral tradition doesn't help his case either, partially from trying to compare it to mere rumors.
The former being correct is also a lot more in accordance with the conclusion drawn from "Tamriellic Faith"
He
might be right, but for the wrong reasons. Akatosh did not exist in the Nordic pantheon until they adopted him from the Imperials (they almost withdrew their support from Alessia, back when they
really didn't like elves), and the Akatosh the dragons talk about isn't the same one (they seem to call him Akatosh for conviencence, and he - or at least his offspring - are a great deal more violent).
Also in The Tale of Dro'Zira, the Khajiit responded to "Alkosh's" Voice... around the time of King Wulfharth, and went to Skyrim. Who was fighting the Nords at this time? The Khajiit may have been responding to Alduin, and that might be evidence for him still being Akatosh.