Do Cittamatra / Yogacara explicitly refute the existence of an external world?
According to Kagyu understanding, Yogacara's position isn't metaphysical, it is methodological. It's not that the external world does not exist, it is that our experience of the world is 100% always represented by organs and/or mind, there is no experience outside of that, so practically speaking mind-made experience is "all we have"*

Which is why trying to figure out how exactly the experience is generated and mastering the practical skills for working with mind and experience is of key importance in Yogacara (comparing to the earlier Buddhism's focus on discipline/ethics and the Madhyamika's focus on philosophy).


  Yogacarins put so much emphasis on the mind because they are concerned with
  practice. Some people have thought they were saying that everything is in your
  mind. Some of these people were influenced by certain Western philosophies.   -- Traleg Kyabgon Rinpoche 


Generally speaking Kaguypas seem to prefer to stay away from philosophical discussions and focus on practice, which makes sense given their self-identification as "the practice lineage":


  We are not concerned about whether or not a chair exists. That's
  secondary. Why spend so much time arguing about whether or not a chair exists,
  instead of learning how we experience and relate to the chair, and what sort of
  experience we've got? -- Traleg Kyabgon Rinpoche




*Using the modern language we could say, reality is an interpretation we make, there is no reality to be experienced outside of an interpretation. In other words, there is no absolute truth separate from the conventional truth.