This cant be correct. Their next statements are they aren’t visible with the naked eye, 5 mm is half a centimeter long, easily visible with the naked eye. 5 micrometers fits that criteria, not 5 millimeters.
https://oceanservice.noaa.gov/facts/microplastics.html
Plastic is the most prevalent type of marine debris found in our ocean and Great Lakes. Plastic debris can come in all shapes and sizes, but those that are less than five millimeters in length (or about the size of a pencil eraser) are called “microplastics.”
As an emerging field of study, not a lot is known about microplastics and their impacts yet. The NOAA Marine Debris Program is leading efforts within NOAA to research this topic. Standardized field methods for collecting sediment, sand, and surface-water microplastic samples have been developed and continue to undergo testing.