You're getting far more microplastics in your diet just from interaction with packaging and household materials than what you're going to get from an ocean-caught fish.
I imagine consuming something that has involved multiple steps of digestion of plastics is probably not the same.
It's a bit like comparing water that I put in plastic container a few days ago to water that's been bottled and sitting there for weeks or months.
I'm not saying there's a difference there, just that at least intuitively it seems that way to me. so a source for such a claim is pretty crucial.