From the article: Even a tiny fragment of aluminum would strike with the same force as a rifle bullet. A hit from debris the size of a golf ball flying at six miles per second could be catastrophic.

Yes, it would except for one tiny fact - the ISS is also moving about 5 miles per second. It's not the velocity per se that makes the impact, it's the relative velocity between two objects. When the Shuttle or Soyuz dock with the ISS, they're traveling 5 miles per second but the relative velocity is nearly zero so no damage. If a golf ball were in a radically different orbit so that it struck with a relative velocity of 5 miles per second, the results would likely be catestrophic. However, that just isn't likely. If they can aim the shot in the opposite direction from the ISS's velocity vector, the golf ball will drop into an orbit with a substancially lower perigee than the ISS, probably causing it to decay fairly rapidly (a golf ball has a relatively modest mass for its size and a high coefficient of drag). Hitting it in line with the ISS's velocity vector would leave it in an orbit with a higher apogee but the same perigee as the ISS. That wouldn't be a good idea but the chance of collision would be small. Hitting it at a tangent to the velocity vector would put the ball into another orbital plane.

The amount of velocity the cosmonaut is likely to impart on the ball is probably pretty low. I doubt if he can get much of a swing while wearing a space suit. If he flubs the shot (likely), the relative velocity between the ball and the ISS will be very low, posing no risk. Even Tiger Woods would have a hard time getting a good hit while wearing a space suit and with uncertain footing.