Hell's official stance had always been "Bayer has never betrayed and has been Zariel's subordinate from the beginning until now."
Amberser knew very little about Bayer, only that after descending into Hell, he had inquired about some of Hell's secrets from the God of Alchemy and the Goddess of Wealth and thus knew that these two had some unpleasant history.
As for the specific details, Amberser was not too clear, perhaps only the two Hell Lords themselves could give a clear account.
In principle, Amberser just wanted to keep a low profile and hoped Melina wouldn't provoke those seasoned Lords of Hell right from the start.
Not to mention someone as sensitive as Bayer.
A Hell Lord who once rebelled against Zariel, succeeded in rising up, only to be brought down by Zariel again and return to being a minor underling, yet still held the title of Blood River Chief General.