Mueller wasn't even appointed yet in Feb, so what was Flynn being investigated for, and if he was, why didn't Comey say so to the Senate committee in March?
Those would be questions Comey would probably have to answer. I doubt the Mueller team came up with the information on Flynn and his son so quickly on their own. Likely, a previous file existed, much like the case with Manafort. That information gave them a place to carry on. Perhaps it was inactive, making the statement true in the technical sense.