No need to speculate. It says right in the article that they used rat poison.
Rat bait was dropped by drone in July 2024 — around 25 kilograms per hectare (55 pounds per 2.5 acres) — in lines across the islands, so that there were no gaps in coverage, explains Jacques. The bait, designed to target rats, has little effect on other species. It needed to be widespread so that each individual rodent ate at least one pellet, he adds.