It all depends on how big the predator is, how big the prey is, how much they consumed and was metabolized before predation. It doesn't magically kill anything that eats it or there'd be dead cats in every street of every city. The older generation of rodenticides were quite lethal to secondary ingestion though, that's for damn sure. Additionally, technically speaking, according to the manufacturer, the rodents are supposed to lose consciousness from the anti coagulant well before death, it isn't a pain inducer.

Also, the article is biased in the same way a company using a rodenticide is in the opposite direction. Sensationalism exists.