They mean to say "90% of the plastic in the ocean that comes from rivers comes from just 10 rivers".

https://factcheck.afp.com/widely-cited-study-did-not-show-95...

"It estimated that 88 to 95 percent of the plastic transported by 1,350 large rivers around the world — referred to by scientists as riverine waste — comes from only 10 rivers, all of which are in Asia and Africa. The study does not attempt to compare the plastic waste transported by rivers to the total plastic waste in seas and oceans."

There's a bunch of plastic in the ocean that didn't come from rivers - mostly industrial fishing waste.