According to one in-universe scholar. Not objective fact.
The source of the Alduin is Akatosh idea is a book titled something to that effect. All books in-game are in-setting and suffer from perspectives, speculation, and misinformation.
As does most lore. All lore before Sky says they are the same. Different interpretations of the same entity. Which makes way more sense than another Aedra running around.
Every race has their own names for most Aedra. Alduin being Akatosh is no different than Kyne being Kynareth.
And what isn't speculation of subjective is the fact that the Imperial pantheon is filled with conglomerated Aedra from other polytheistic followings. Further supporting the fact that these are all the same deity. Just like how you get similar ideologies (great floods, etc.) around the world in real life.