They may not have. DMs are imperfect, too, and it may not have been communicated. There’s also the problem where D&D is a game and people will have certain expectations. I’ve got our DM to now include the occasional meta note when it comes to changes in how the world works mechanically so we don’t fuck everything up based on previously set precedent.

If the DM had never shown them suicide bombing NPCs and this was just a mage who happened to have fireball(i.e.: not a fire mage) and then went straight to a no-warning TPK then buddy can get fucked. If that setup was in place, then it’s party’s fault. We don’t know this from their comment.