The link is broken so I can't read the article. In any case, I'm surprised to hear the WT flatly condemning slavery. Biblical references to slavery, old and new testament, are unilaterally positive. Not even Jesus seems to have condemned the practice (at least not directly).
I wonder if this is just due to ignorance (unlikely) or the beginning of a less fundamentalist position. At this point it's only tacit, but if the GB could just concede that the Bible contains errors - both ethical and historical - it could begin a liberalizing trend for Jehovah's Witnesses. Who knows, 50 years hence there might be openly gay elders and teenaged couples holding hands in the Kingdom Hall.
Wouldn't it be grand?