Mechanics of the game dictates that poisonous creature:
1. attacks normally, THEN
2. If the attack goes through, apply skill
In this case, the skill is "venom" (Forest Scorpion and Puffer Fish), Deadly Venom (Deathstalker), or Neurotoxin (Dune Scorpion).
Criteria of attack going through:
1. Dealing damage. Negative damage (healing, via AM) is counted as damage.
2. Not fully blocked by shield. (Blessed Deathstalker deal damage through Perma but not Diamond, for example)
3. Not fully blocked by creatures. (via GP, as mentioned)
EDIT: as for dealing poison if devoured, it's not Venom. It's passive skill Poisonous. So a different skill.