It's the finding that brain tissue seems to concentrate microplastics at a much higher rate than other tissue in the body

If I remember correctly, the method they used to detect microplastics, which involves pyrolysis, gives much the same result for lipids (which brain tissue has a lot of) as pure hydrocarbon plastics like PE and PP, because they all feature relatively long hydrocarbon chains and the pyrolysis products will contain the same short-chain hydrocarbons.

I find it concerning that there seems to be such a concerted effort to downplay the significance of that finding

There is nothing to be concerned about. This is just the (re)discovery of basic chemistry and the natural response to misguided alarmism.