My guess is that he is simply taking the high road. Eventually, whoever is behind this ad (besides NIMROD) will be brought to light and embarrassed...badly!
The magnitude of this out-right lie is such that it would not be at all out of line for him to call bullsh*t! They are saying that he supports people killing pro-choicers!
I wish but when has any dishonest, fabricated or corrupt act embarrassed anyone on the left? Why would it nobody reports on it, it just goes away.