Probably concerned about worst case scenarios. This is a bioengineered yeast that eats PET. What if it populates your gut and fucks up your gut fauna and hurts you?
What if you shit it out and it gets into some waste area with lots of plastic and starts multiplying uncontrollably?
What if it evolved to eat other plastic and now all our current plastic starts literally molding?
There's a lot of weird shit that you can be concerned about. Might be low risk but low risk of extreme new scenario.