Will plausible deniability mean no reaction to North Korean attack?

The New York Times is also carrying an article on Seoul's response, including President Lee's call for military strikes if there is any further provocation. The NYT also notes what could be called North Korea's deniability factor (which usually revolves around denying they did anything or saying that they did something because they were provoked):

The North blamed the South for starting the exchange; the South acknowledged firing test shots in the area but denied that any had fallen in the North’s territory. It was in the same area that a South Korean naval vessel, the Cheonan, was sunk in March, killing 46 sailors. Seoul blamed a North Korean torpedo attack; the North has denied any role.

The problem with the idea that North Korea was simply responding in kind to a test shot landing in North Korean territorial waters is that means they responded by bombarding civilian positions, not the ships taking part in the exercise who allegedly would have fired the errant test shots. It simply doesn't hold water.

Meanwhile, some in the South may find comfort in thinking North Korea wasn't behind this, because then they would feel better in not taking some of the tougher choices described in many news outlets, including the Christian Science Monitor.

I've already proposed the Kushibo Plan, but if Lee takes too long to respond, I'll have to devise a non-military massive retaliation.