gnolls, are not a pruduct of a dead hyenna.
I am quoting from D&D 5E Monster Manual, p. 162: "In much the same way Yeenoghu created the first gnolls, a hyena that feasts on a fang's slain foe undergoes a horrible transformation, becoming a full-grown adult gnoll."
This is what they were trying to depict. The hyenas ate the flesh of the dead humans that were slaughtered by the gnolls, amongst them also a gnoll fang of Yeenoghu, and so they underwent the transformation. It's not just a dead hyena, there is a demonic influence involved.
Such a thing as Baldur's Gate Series lore does not really exist. It is the Forgotten Realms lore which is established by D&D rulebooks, novels set in the Forgotten Realms (e.g. from R. A. Salvatore) and official videogames such as Baldur's Gate, Neverwinter Nights, etc. Likewise this game's story will become part of the official Forgotten Realms lore.
like the letter from volo to elminister ( what could not be possible because of the timeline larian talked about )
Elminster is still alive. He is one of the most powerful wizards - if not the most powerful wizard - in the Forgotten Realms and a chosen one of Mystra, the goddess of magic and she keeps him alive.
