I too have been frustrated by this passage. I contribute this to the characters' mistaken assumptions. I think it was RJ showing us that the characters do not know everything. Alternatively, at the time of this statement it could have been true, but not true now?

Also, don't forget that Nynaeve is held by the two black ajah in Ebou Dar, but once she breaks free, she evenly matches against the two. That says a lot about how strong Nynaeve was at the time.