Combined with the previous two episodes, it's also a demonstration of why it's hard for even really experienced players to get good at D&D. Like, the collective behaviour of the group, when judged in terms of "we are in a very dangerous area and are trying to stay alive" was, let's be honest, really fucking stupid. And we're 45 episodes into the second campaign.
The problem is that, unlike in a videogame, when the PCs die the players don't get to restart the fight. They don't get to try again and again with different tactics until they learn how to play better. So DMs shy away from giving them super deadly challenges that require really good play in order to win, in order to avoid having a completely new roster of characters every few sessions.
But players don't learn much from encounters that aren't challenging. A band of 5 goblins versus the M9 right now would get wiped in a single turn no matter how inefficiently the party went about doing it. Players also rapidly get bored from those encounters. Deadly fights are fantastic because they require a high level of player engagement in order to be overcome. When players realise they'll need to pull out all the stops you see them lean in, ask the DM for more details, paw deeply through their character sheets, think hard for some unconventional angle of attack.
But by the time they realise they're in such danger and start really paying attention, the fight has already begun, and probably begun badly because of it. They're forced to try new tactics, but only after wasting time before realising their old tactics weren't cutting it, and there's only so many times they can try new things before getting wiped out. And they all have no idea what their fellow party members are going to do either.
You look at the M9 during the fight with the hydra and this dragon, and they look like a bunch of students cramming the textbook while waiting in line for the exam.
For comparison, look at how they behaved after Molly died, when infiltrating Lorenzo's base. They fucking SWATted that shit, tactically scoping things out, keeping in communication, they were tight. And now, many episodes later, they've gotten too loosey goosey again.
They need to write up some kind of informal document for group tactics, starting with never split the party in a dungeon, goddamn.