You suspected he was still lying and pushed into a position where he revealed information he clearly did not want to.
Sounds pretty smart to me. Keep the pressure up, it's the only way you'll get to the truth. If he admits that now, just wait until he's standing in the car park outside the polygraph.
In the Uk there's a fairly senior polygraphist who I have spoken to that reckons the polygraph really just adds a little confidence to the proceedings, and the car park is where the real truth is often exposed. Pathetic really, isn't it.
You on the other hand, are not pathetic. You are kicking ass for your marriage. Keep up the good work.