Thinking of poisons in binaries is false - the dose makes the poison. You are eating "poisons" every day, and it is impossible not to. Plants create poisons to defend their seeds against herbivores, but you don't die when eating nuts or grains, because your body has adapted. Water will kill you if you drink too much of it. So the question is not "is this a poison", but "at what dose is this a poison".
You could argue that with food, there is a necessary trade-off, but with PFAS there isn't, because we don't need it to survive. But that is true for most of modern civilisation, and what you then end up with is the "this substance is known to the state of California to cause cancer" on basically everything.
You are right that this becomes a difficult problem of epistemology. Basically, it is impossible to prove "this is safe", only the opposite, and often it is not easy to prove conclusively. I'm not sure where the right balance lies, either.