In landfills things stay buried if/until they can't get washed away. That's how microplastics get out, but bcs of the filtration of the ground that is very very slow.

And paper actually decomposes, usually with bacteria or other oxidation precesses. Plastics don't really do that. If they did there literally wouldn't be a microplastics problem. Something that smol would "decompose" basically immediately.
(Before bacteria ate celuloze trees would litter everything, basically a catastrophe, and then they became fossil fuels bcs there was nothing to decompose it.)

But it's a hella convenient way of thinking that plastics in a landfill aren't a problem.
Unlike cellulose, plastics do degrade into microplastics.