But surely this process you outline implies that the PFAS already there was deposited in the (distant) past and is no longer being deposited in any substantial quantity, otherwise the problem will just reoccur (build up over time)?

Meanwhile most indicators I've seen imply that PFAS contamination is -in some ways- getting worse, and is not only from historical sources.

Also, earthworms will mix up deeper and more shallow layers of earth and plant roots will extract PFAS from deeper layers and deposit them on or near the surface when the plants leaf matter dies off (in fact I know of several types of plants including hemp being successfully used for this exact purpose).

I'd love to find definitive proof that PFAS can be remediated on a hobby garden scale (in one's own backyard) but I'm skeptical of the success rate and longevity of the endeavor...