It was Medium, and on the easy end of Medium at that, but this isn't what Hard means either. Hard means you might have to use some resources, yes, but only if things go poorly or you want to get through it faster. If you want to talk about burning all resources, then you have to consider things like a PB level 5 Fighter by themselves with nothing but a longbow and 4 arrows kills the average Black Pudding in 1 round if they burn Action Surge + 4x Superiority. No, a Black Pudding doesn't warrant that kind of need. It warrants limiting how you split it to the expected 1 accidental and up to 3 strategic and engaging the easy to hit and slow ooze from a range, but that's about it.
This was a display of players waking up in a burning building and, because there wasn't a fire extinguisher, they started throwing gasoline around the room. Yeah, D&D can be mean about punishing that sort of thing, but I'm not sure there's a good fix. Adam didn't have any reason to believe they wouldn't just kill it in 1-2 rounds.