The fact that they refuse to get certified / conduct any independent rigorous testing (like in NSF/ANSI certification) should have clued you in.
"We also cut open a pair of Black Berkey filters to see how they are constructed and to look for evidence that, as Berkey marketing claims, they contain “at least” six different filtering elements. We found that though the Berkey filters are larger and denser than filters from Brita and 3M Filtrete, they appear to share their filtration mechanisms: activated charcoal impregnated with an ion-exchange resin.
"
I wouldn't count on any of this to filter chemicals, much less PFAS.