I don't know the details here for PFAS (and they likely would vary enormously for the different molecules that fall into this broad category). But in general a molecule doesn't have to react to be accumulated. Inert usually means it doesn't react with other substances in a normal environment. It doesn't mean you can't make it react if you add enough energy. For example nitrogen gas is considered inert. Bacteria (or chemical plants) can make it react and produce different nitrogen-containing molecules from it.

Inert doesn't really say anything about toxicity, it's not directly related to that. The opposite is though, pretty much any strongly reactive chemical is dangerous or toxic in some way since it will react with stuff humans are made from.

With PFAS the inert example is also usually Teflon. That is also a solid polymer, so not many individual molecules. There isn't much you body could do to process a macroscopic chunk of Teflon, so you'd almost certainly just excrete it.