This is clearly fantasy lava that is not too dense. Note that the adventure text consistently refers to falling (or even plunging) into the lava.
If the designers had intended for this to be realistic lava that obeyed Earth’s physics, then surely they would have said onto the lava.
Yup. That’s why I said up-thread that I would consider
“the lava” in the adventure text to simply refer to the area at the bottom of the chasm. That’s what the characters fall into. I think you’re right that the writers seem to have been imagining fantastical lava with a water-like consistency, but then they also must have been imagining it as much cooler than the roughly 1,500 to 2,200 degrees F lava that erupts from the interior of a planet like ours. Even boiling water at 212 degrees F is likely to kill a person dropped into a pool 20' deep by 30' across if they can't manage to get themselves out within the few seconds it would take for their muscles to begin to cook or have someone pull them out. The burns would probably kill them eventually anyway. Whereas the writers have assigned a 10d10 damage expression which according to the Damage Severity and Level table on DMG p. 249 is a dangerous rather than deadly amount of damage for a 13th-level character which they're likely to survive as long as they can get out within a round. Also on p. 249, as cited above by
@plisnithus8, the Improvising Damage table gives the example "wading through a lava stream" for 10d10 damage, while "Being submerged in lava" is given as an example of 18d10 damage, a deadly amount of damage at 13th level.
Deadly in this case still doesn't mean it necessarily kills the character unless they're already wounded.
So there seems to be a discrepancy between the situation implied by the adventure saying the characters plunge "into" the lava and the damage they take when they do. One way to try to resolve this discrepancy is to increase the damage. By using the maximum damage, you essentially increased the damage expression to the 18d10 deadly damage that the DMG gives for submersion in lava, but as you saw, this still wasn't enough to kill the characters, which is why I said if that's the fiction you wanted, just skip the hit points and kill them. Another way is to recognize that the adventure writers meant the damage to be merely dangerous rather than deadly and to adjust the fiction around the situation accordingly. This is consistent with the lack of a saving throw or any way to detect the trap, and there are lots of possibilities here. One is that, even if you're imagining fantastical lava that the characters could sink into, it needn't be uniformly water-like in its consistency. Firmer, cooler patches and floating chunks could exist, and a damage result that doesn't incapacitate a character could represent the luck of landing on one. When exposed to air, lava forms a thin crust which might be enough for the adventurers to run across if they're lucky. Regardless of the details, I think it's up to the group to come up with an explanation for why or how the characters escaped with their lives.
I really don’t think this conversation is going anywhere fruitful at this point. My next session is tomorrow night. We will most likely finish this dungeon floor and move on to the next one, and it will all be lava under the bridge. Thanks everyone for your contributions!
I hope the session goes well. The conversation provoked some interesting thought on my end, so thanks!