I'm sure Ross and the ABC news staff in general made the natural assumption that anything coming from Mueller would dovetail with the left-wing narrative, so they reported it that way. Their absolute belief in the narrative made the inaccuracy seem like an unimportant detail, marginally worthy of a "clarification". They were undoubtedly pleased by the stock market plunge. It was only the totally unexpected media reaction that took them by surprise and finally forced the reluctant "correction" hours later. Then the boss has to put on a phony act of outrage not so much over the inaccuracy itself but over the ammunition it hands to Trump accusations of left-wing bias and fake news.