Actually, seems they used the plastic bottles only, but didn't specified this clearly:
"A possible explanation is that some particles of this kind are newly released from the bottle package during transportation or storage, which are retained faithfully in the water sample."
but many was not introduced by package:
"Other polymers such as PA, PP, PS, and PVC, which are not the packaging material but also identified with significant numbers, are most likely introduced before or during water production."