Their explanation for hard water boils down to (pun intended) the plastic particles serving as nucleation sites for calcium carbonate (lime scale) precipitation when the water cools, although they don't present it quite so scientifically and they somewhat gloss over the fact that this doesn't remove them, it just makes them easier to filter out. I suppose that depending on the vessel material and how it is cooled, these precipitates could form on the vessel wall, effectively removing them from suspension. They don't present it, but I suppose a similar process, i.e., the particles adhering to the vessel wall on cooling, through one mechanism or another, might explain a reduction in suspended plastics in boiled and cooled soft water, but whether that is a practical ongoing removal method or not is another matter.