It's not so much that it's a secret (or a fear of others knowing), but rather that it's none of the other person's business. They have their own path and the Wiccan or Pagan has theirs. To interfere with another's path by telling them something they may not want to know is contrary to Wiccan and Pagan beliefs. (Although, reading more of that article - maybe it's an American thing? "The auditor said that by �coming out of the broom closet,� he risked ostracism at work and perhaps being pushed into early retirement, which would affect his pension." Wow.)
Actually, this is an interesting topic - how far do you take the "No prosetylising" guideline? If someone asks about your beliefs, does that mean the topic is open for discussion, or do you still feel reticent about saying too much too soon?