This is a guess but I would assume the bottling process in water bottling plants, and the manufacture of the disposable water bottles, contributes to the amount of microplastics more than passive decay of plastic. Really my main points/beliefs are:

1. We should be careful making claims based on scientific studies to make sure they are accurate to the study, especially when it comes to claims about how a solution for a problem may be reached. A slight misunderstandings can cause good motivations to make things worse (like people collectively throwing away all their reusable water bottles and buying NEW water bottles made with metal, effectively turning millions of usable waterbottles into trash and creating demand for more polluting industry).

2. Plastic pollution, microplastics, and everything related, is an overproduction industry problem, not an individual responsibility problem. While a concern for ones own health is individual, it's also almost impossible to meaningfully avoid microplastics with the current situation. The responsibility doesn't rest on the shoulder of consumers to collectively make good choices, but on governments to regulate and for owners of industry to be held accountable for the damage they have caused.