I don't know why he's still alive, however, Prussian blue is the antidote for thallium poisoning.
So the "secret FSB department which specialised in locating and liquidating people considered a danger to the state" is tasked to locate and liquidate a former KGB colonel who knows KGB poisons, as cited by the article. Their method of murdering him is thallium. I had to look it up - a slow acting poison (taking days and weeks to kill) with early onset of symptoms and a widely known antidote (Prussian Blue).
Thallium poisoning is obvious. It wouldn't be mistaken for food poisoning or a heart attack. If the killers weren't concerned about that then why not use one of the hundreds of poisons available to them that have NO antidote?
Maybe in deference to his ex KGB rank, they just gave him a "warning".