Garbage can sometimes be identified. Think floating bottles with intact labels, yoghurt containers with a specific language, etc. anything so tiny that it can’t be identified is almost by definition part of the global flow because it takes time for plastic to break down.

... and locals are using this logic?

...why wouldn't they be?

They are all commenters on hacker news.

I'm not caught up with the microplastics problem.

But I suspect the locals had read testimony from scientists studying the problem. People really give a shit about pollution in Hawai'i.

It also seems very unlikely that the quantity would be there without ocean currents bringing it in from the big offenders like China.

Hawai'i has a pretty small population of people who are widely very concientious about polluting the ocean, compared to most other people in the world.