How so? From the article:
a modern, well-run sewage treatment works removes 99% of these fibres from wastewater. So far, so good. But [...] In the UK, of the sewage sludge screened out by treatment works, 87% is sent to farms. The microplastics so carefully removed from wastewater by the treatment process are then spread across the land in the sewage sludge the water companies sell to farmers as fertiliser.
Either the figures are totally made up, and/or plastic in the environment—on the fields, on the produce, washed into the rivers and the oceans—is harmless, or else there is cause for concern.