If someone believes slavery is consistent with liberty, he is obviously wrong. If you think this article argues differently, please reread.
You cannot use anything in this article to argue for the rightness or wrongness of any particular views: that is not what this piece is about. It is about the way in which views are held and shared.
So, to apply the argument in the piece to your example, I would say only that an orthodoxy that held that slavery is compatible with liberty would not be inhibit someone who held that view from seeing that he is mistaken. If he held that view with an open mind and willingness to improve his understanding of liberty (rather than believing that his own view is the standard for testing others' views of liberty), he would be able to see the error of his view and change his ways. Moreover, that willingness to improve and learn would make him a more effective advocate for liberty thereafter.
Hope that clarifies. Thanks