I think that article is based on outdated information. More recently people have started to look at plastic released on the open sea through fishing, and found it to be bigger than all land-based sources.
Well that's some confusing reporting. The title is "Dumped fishing gear is biggest plastic polluter in ocean, finds report" yet does go on to say 10%.
Greenpeace: "Ghost gear makes up an estimated 10% of the plastic waste in our oceans, but represents a much higher proportion of large plastics found floating at the surface."
https://www.dw.com/en/almost-all-plastic-in-the-ocean-comes-...