Remember: Kim Jong-Un’s brother was not killed directly by North Korean goons. They hired two women they convinced they were working on a prank show to spray him with the poisons.
After reviewing the video tapes the police concluded that the women knew that they were handling poison - they kept their hands away from their body and immediately washed them after the attack.
Someone could have told them it was anything else that you wouldn't want on your body. Like, fart spray or whatever. A prank. That behavior doesn't really tell you anything conclusive, but I guess they just let anyone be a cop these days.