If they have worms (FEC test) - then just use a wormer from the vets. No natural wormer will clear an infestation. If they are clear now and the ground was clean when they came on, they shouldn't have any problems in the future, as no worm eggs to pick up in the first place.
Wild goats are not limited to a small area of grassy field, they roam acres and acres... so of course only minimal worm egg pick-up. Wild goats will also preferentially browse branches/bushes etc rather than graze grass.