Shouldnt we first know the reason he is alive, before we judge it as stupid?

I have not read the comics. But as I understand it Minsc & Boo were turned to stone at some point after BG2. Then healed back to normal a little bit before Descent Into Avernus I believe(not sure how/why/when exactly).
Personally I don't have a problem with this. It is 100% possible to happen in this universe...hell we fight Drow that have been turned to stone for longer than Minsc & Boo in the Spectator fight in the UD.