Link or it didn't happen
The producer didn't say he was stupid, but they did say he was ignorant regarding the substance of Ramsay Bolton.
Its not that surprising really. Ramsay was a relative unknown until recently, so its not surprising that LF wouldn't have a "file" on him the way he might on others that are more well known.
But I don't think he really cares that much. He sees Sansa as the key to the North, just as the others do. I doubt he will be all that broken up over what happened.