It's amusing that you think you can decide what is and isn't a general principle, from the same line of text, in the same table.
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Yikes.
What do you describe happening to your fighter when he is immersed in lava?
I think Theodoxus was giving an opinion that some things should be better handled in a way other than HP damage, not that he expected a table of improvised damage to explain how to run lava differently.More generally, the fully realistic way to run a lava flow is a little bit beyond the scope of the improvised damage section. There are already rules for suffocation and toxic fumes, so I think expecting a table giving guidance on how to make up damage numbers in a pinch to go into detail about how to run dropping someone into a volcano is a little unfair.
I do wonder how all of these things are being misconstrued consistently...