From the article, under "Unfollowing": "You have to unfollow the people you follow for two reasons. The first is that you cannot be following over 7,500 people at any time. The second is because — although artificial — you want to have your follower/following ratio as high as possible as it is a sign of a more desirable account."
You have a limit of 7500 people you can follow. Thus if you're going to be spamming following people, you need to unfollow them as well to make room for more follows.