That's my guess as well. The dialogue mentions the power of a chosen which would fit. As would an old avatar. If it were Jergal himself he would be disguising his form and faking his time in the tomb (Jergal looks like a praying mantis.)
again datamined scene synopsis
"After the party has slain a group of undead scholars, the judge of death--Jergal--climbs out of his sarcophagus to greet them. He has been locked inside for years by the god Helm to atone for his part in raising the Dead Three, wicked gods who are now plotting to conquer the realms. Jergal is emotionless and excessively formal, both resigned to his fate and that of others. He is here to assist the party because that is his divine oath, not out of any true desire to act. Players can attempt to goad or attack him, but to minimal effect, as he is functionally immortal. His only goal was to see their faces, so he can find them at camp later to offer further assistance."
"Jergal, a god of the dead, awaits the player in camp. He is bound by the judgment of another god - Helm - to assist the players as penance for past crimes. He provides resurrection services to the players."
That contradict ~30 years of Forgotten Realms lore. I would be rather surprised that WoTC would just allow the devs to change that. Even more curious, the place where we find him was a Selûne's chapel just a few months ago (in demonstration) and everything related to that is still in the game files.
