Nice to know.
You aren’t CARRYING the poison, are you? If you carried around enough poison to make, say, ten DEEs… You’d already be carrying a good few gallons. An actual SPRAY of poison? You’re either making that on the spot, or summoning from some magical poison storage bin.
Yes, poison in general existed, but the specific poison you are using has to come from SOME where. Nightblades can at least just put poison on their blades before they leave home, but Occultists can’t pre-make DEEs enough to do all their murdering before they leave home.