Modern US landfills are lined and capped water tight, and have been for decades. Permitting is a thoroughly reviewed process, by dozens of federal, state, and local government offices, and hundreds of officials and engineers. Monitoring wells ensure compliance.
Even if it's microplastic in that landfill water, it's not getting to the ocean. Worst case is it's captured with the decomposing gases and condensate, and reprocessed at a refinery or incinerated.
The plastic is much better deposited in US landfills than shipped overseas, where it may be directly spilled into the ocean, or dumped into open fields for the poor to manually sift for the most valuable material, and then there's no telling where the rest of it will end up once it rains.
Plus landfill plastic is sequestered carbon.
Even places that certify plastic recycling into a new end user product are often making worthless items that are given away so that they can claim zero waste. The material ends up as construction fill for hydraulic detention infrastructure, playground surfaces and sports fields, or even those "green" children's toys. Or cheap fleeces. Lots and lots of cheap fleeces, blankets, and snuggies.
The exact places we don't want that plastic to end up.