Polystyrene itself isn’t toxic but it degrades on exposure to UV light (the stomach acid and digestive enzymes of the animal probably does a number on it too), and when it breaks into microscopic particles it’s known to be toxic: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7193629/

There’s also likely still traces of solvents, plasticizers or other additives that aren’t meant to be ingested, especially if the polystyrene wasn’t manufactured with food safety in mind.