I'm not sure how popular that claim is, but I've heard it multiple times. Especially as part of the eco-protective initiatives.
I was able to find this example[1]:
"Even if you live hundreds of miles from the coast, the plastic you throw away could make its way into the sea. Once in the ocean, plastic decomposes very slowly, breaking down in to tiny pieces known as micro plastics that can be incredibly damaging to sea life. 80% of plastic in our oceans is from land sources – but what does that really mean? Where is it coming from?
There are three main ways the plastic we use every day ends up in the oceans."
"1. Throwing plastic in the bin when it could be recycled. Plastic you put in the bin ends up in landfill. When rubbish is being transported to landfill, plastic is often blown away because it’s so lightweight. From there, it can eventually clutter around drains and enter rivers and the sea this way."
"2. Littering
Litter dropped on the street doesn’t stay there. Rainwater and wind carries plastic waste into streams and rivers, and through drains. Drains lead to the ocean!"
"3. Products that go down the drain
Many of the products we use daily are flushed down toilets, including wet wipes, cotton buds and sanitary products."
At no point they mention the fact that some countries are just using ocean as a landfill and that causes 90% of the pollution of it.
[1]: https://www.wwf.org.uk/updates/how-does-plastic-end-ocean