Degradation Rates of Plastics in the Environment (https://pubs.acs.org/doi/10.1021/acssuschemeng.9b06635) shows PET degrading very slowly in the environment. _Very_ slowly.

Researchers have found bacteria that do degrade PET using esterases though: https://www.science.org/doi/10.1126/science.aad6359 and https://www.sciencedirect.com/science/article/pii/S016041202...

So I guess technically it's biodegradable? Though as it's an energy source give bacteria a few hundred years or so.