you are misreading the context. According to Winship, Piketty's results are wrong/irrelevant for other, independent reasons. But not everyone shares this view

you are misreading the context. According to Winship, Piketty's results are wrong/irrelevant for other, independent reasons. But not everyone shares this view, so Winship is clarifying that, for people who were initially convinced by Piketty's analysis, the FT criticism is important. But it is not important for Winship because even without the FT criticism, he has independent reasons to discounting Piketty's analysis.