FreePDK is not a "real" cell library in the sense that the core logic cells it defines are not real ones that can be manufactured/fabricated by any entity. It is a fake kit that can be used to both "estimate" chip area (for your own RTL, for some definition of "estimate") and also used as a starting point if someone wanted to design their own PDK (e.g. VLSI research into new PDKs done by research labs.)

The fact that this PDK will eventually be "production ready" for a real fab and real chips can be printed using it will be a significant step forward.