I’m not 100% convinced this is that problematic though. You floss and then rinse/spit. And of course this would only need to get rid of the quantity actually shed from the strand. It would be interesting to see the load actually consumed on average, but my gut says it’s probably irrelevant.
The real issue is the unchecked spread into our environment, causing us (and everything else) to unavoidably ingest it regularly and without limit.
Edit: To be clear, my general position on PFAS is “broadly enact bans on its use out of sheer caution” due how rapidly it’s turning up in the environment. This would be no exception. I’m just saying I’d be surprised if its presence in dental floss actually contributed meaningfully to your daily load given the amount you consume incidentally due to widespread environmental contamination.