Those microplastics aren't coming from landfills. They're coming from your everyday interactions with plastic and people dumping plastic in rivers and oceans.
Maybe not the landfills once they are buried. But certainly from waste transfer stations or unburied waste, seagulls and other birds will pick up trash because it smells like food or may have some food in it, and then drop it outside of the landfill/transfer station.
If you don't like microplastics, then you should be pro landfill.
A very large fraction of those are from plastic waste in Asian countries that gets dumped into waterways instead of being landfilled.