My simple explanantion for what is going on:

Keratin-18 is used as a marker of circulating tumour cells in the body. So, zombies are having increased rate of mutations resulting in tumours. Follistatin suppresses tumours, so their leader (W. I. Roberts, of course!) is supplying them with follistatin to reduce the rate of tumours.

It may also be possible that follistatin and zombie cancer is unrelated; and follistatin is used to increase muscle growth in zombies.