As far as I can tell, they currently don't.

  “How will you deal with context?”

  We are currently developing a principled method to identify 
  relevant phrases, for example to deal with the multitude of both 
  positive and negative uses of profanity. We expect to be scoring 
  phrases instead of words, where appropriate, in the near 
  future."

Also, they mention elsewhere that they use a bag of words classifier, so they can't really account for things like this easily.

I wonder how they deal with ironic statements. It seems that they merely extract words out of the context.

They don't. But consider that a good bit of those would be neutralized by phrases like "not sad" etc.