If this is Chinese research it is probably sloppy and likely fraudulent. The “5mm” error certainly indicates the first point is correct.
Microplastics are fragments of any type of plastic[1] less than 5 mm (0.20 in) in length, according to the U.S. National Oceanic and Atmospheric Administration (NOAA)[2][3] and the European Chemicals Agency.[4] They cause pollution by entering natural ecosystems from a variety of sources, including cosmetics, clothing, and industrial processes.
https://en.m.wikipedia.org/wiki/Microplastics
Plastic clothing fibers would make this up, too. Take a look at the dust around you or on your furnace filters, and you’ll see a bunch of lint, made up of skin cells and mostly small clothing fibers.