considering Adagium is one of Lucis dark secrets thats probably why the invasion was never mentioned, they tried to hide or cover everything about it as much as possible.
if the true story of what happened would come out it would shake the trust of the people, meaning the dinasty they followed for millenia was built in treason, that the gods allowed a murderer to be the worlds savior.
Inconsistecies aside I simply love the added layer on the story. Now Adagium story can be told.
The funny thing is, if the treachery wasn't a secret, it probably wouldn't have done as much damage.
Like, Somnus' closest real-life parallel would probably be King David, whose successor was born to a woman he had taken from her husband by intentionally sending said husband off on a suicide mission. And that's in the official records!
Of course, the problem with Ardyn is that his existence is better hidden for reasons other than Somnus' reputation. If Ardyn had actually died, though, Somnus being honest about what happened almost certainly would have been better in the long run.
EDIT: Waaaaaaaait a second. I might have figured Somnus out!
So, for most of the episode, Somnus is cold and pragmatic. He does whatever he thinks he has to, without letting inconvenient emotions or empathy get in the way.
But then, when he confronts Ardyn, he changes completely. Instead of his usual cold, rational mask, he smirks and struts and makes wild false accusations in front of as big an audience as he could muster, attacks his brother as if he'd been waiting for the opportunity for ages, calls the murdered Oracle "foolish woman," and teams up with his shield to continue slashing at Ardyn even after Ardyn turns his back to them.
...and, once the witnesses are gone, Somnus' expressions go pensive and blank again.
What if Somnus wanted his people to think that his rewriting of history was the result of him being a terrible person, so that no one would ever have any reason to consider that Ardyn might have been hidden because he was still dangerous? That'd certainly explain how different he seems in that scene.