This is a weird move on Cohen's part. I've seen even many Trump supporters acknowledge that the affair probably did take place. Nobody was exactly shocked to hear that Trump may have cheated on his third wife in a row. It looked like the story was on it's way out of the news cycle until Cohen's statement.
It is a weird move. I'm still on the fence on whether it was pure incompetence, or a "strategy" on the part of the White House to flood the newscycle.