The author thinks that responding militarily could give North Korea the excuse to go to war in a limited way to preserve their regime with a foreign war. The way to go is to squeeze North Korea more with economic warfare.
He has a point on the latter issue. With North Korea imploding, the best thing to do is to hasten the winning strategy and not risk a loss or high-cost military victory by escalating to arms.
But I think his worry that a South Korean military response could provoke a war is not valid. North Korea needs no excuse based on actual South Korean attacks. If North Korea wants to attack South Korea, they'll just tell their people they were attacked first by South Korea.
As I noted early on, a proper South Korean response that uses military force should be equally deniable in public. The point of retaliating quietly off of the main front on land would be to send a signal to the North Koreans that Seoul won't be effed with at no cost--but that Seoul isn't contemplating regime change military action.
No need to rub Pyongyang's nose in the doodie to compel them to escalate to save face. Just do something fairly quiet that shows the northern elites that the south won't be pushed around.
And continue to talk and squeeze North Korea until they die--which has long been my policy preference.
UPDATE: Was it a small semi-submersible?
The unidentified officer said a North Korean semi-submersible vessel carrying 13 crewmembers fired a torpedo at the Cheonan, according to Choi Sung-yong, who said he had spoken to the officer by telephone several times in recent days.
Ramp up the economic warfare against North Korea, and quietly make something blow up in North Korea that can be denied by Seoul.