I don't think that is a helpful comparison. Of course being 'inert' is not a good heuristic for harmless -- a bullet made out of teflon fired into you will harm

I don't think that is a helpful comparison. Of course being 'inert' is not a good heuristic for harmless -- a bullet made out of teflon fired into you will harm you quite severely despite teflon's inertness. My implication was that without further evidence there is no reason to assume harm by default, because inert things are non-reactive and there is no known mechanism for microplastics having a mechanically harmful effect on mammals.