Looking at the Twitter remark, it does obviously look like sarcasm. Especially when you've seen him interviewed on camera before - him saying something like that in a serious manner is a laughable accusation. Unfortunately, stuff like that doesn't transmit over text so well. Still, a bit ridiculous for anyone to have assumed he was literally calling for all Muslims to be slaughtered, let alone a journalist.
Sensationalism like this reminds me of the time Huffington Post tried to claim Glenn Beck supported slavery and then posted the supposedly incriminating audio... Which had him saying exactly the opposite.