I agree with you that ideally he should focus purely on Hillary and policy.
The problem was that interview with George S. who asked him, and the media kept raving about the speech. He should have kept it policy related and moved on. I typed up a sample statement on FR. Trump should have had something like that prepared. To be fair, he did put a statement out, but it was after the hoopla started.
Yes, way unfocused. He does have the ability to come back, lets just hope he uses it.