Making that ultimatum to a country being bombed by the US is silly and unreasonable. Only if there is an evidence of Saddam being involved in those attacks, a retaliation would be justified, WITH OR WITHOUT any cooperation. Saddam can be relied on to continue to defy America, and neocons know it.
Buchanan is pointing out un-American interests in the neocon position and you are free to point out why a total war in Middle East is in the American interests.
I don't see any evidence of Buchanan adjusting his views to be just the opposite to the neocons', like you suggest may be true. What I see is a consistent position in both camps. The neocons' warmongering was predicted to happen right after the bombing. The motive is an important part of any analysis.