Maybe there was more to the interview than what was included in Horowitz's piece, but looking at it, it was Matthews who brought up Kennedy, and Coulter at no point in the exchange said anything remotely resembling an "attack" on Kennedy. All she said was that he was a weaker president than a Republican would have been.
So, what exactly are you talking about?
After praising him to the gills in her book. She never said he was "weaker." Yes, saying that he was a weaker president is an attack. It may be a justified attack. However, it doesn't match up with what Coulter allegedly wrote about Kennedy.