Except "countenance" is not to "cheek" as "toxin" is to "poison".
Nor is "ruddy" to "blood" as "toxin" to "poison".
Countenance does not mean cheek, it is the entire face. Ruddy is a healthy red, whereas blood is a substance - not a colour.
All toxins are poisons, but not all poisons are toxins, not the other way around. Radioactivity, as you suggested earlier, is a poison to the human body, but it is not a toxin.
Amusingly, Oxford says of toxins "An antigenic poison or venom of plant or animal origin", which only further establishes ground for the toxic Acheirai gas to be classified a poison.
Prior to (and after, now) my military service, I was a scholar of the English language. If you want to argue meanings and hard definitions, I can do that with you all day.
However, what this is really about is the -spirit- of the rule (since no hard definition can be found in the D&D universe), and it's hard to argue that the Druid of sufficient level should not be immune to a naturally secreted toxin from an animal, regardless of it's planar origin.
Of course, I am not the DM of that game, and if the DM says "Roll a fortitude save to resist the toxic gas," I roll a fortitude save and talk to him about it after the game.