Are the tars in cannabis considerably more dangerous than those in tobacco?
This is not a complete answer (merely a step in a particular direction, namely that of the falsehood of the claim), but I contacted the author of the book (Introduction to Medicinal Chemistry) Graham Patrick and he wrote the following:


  Dear Jori,
  
  Many thanks for your e-mail and observations. Unfortunately, that
  statement was made in a very early edition of the textbook published
  over ten years ago and so I doubt if I will be able to track down the
  original reference on which it was based. As you say there is
  controversy and the statement is too sweeping. Work on the 6th edition
  is starting this year and so it will be possible to modify that
  particular passage. Many thanks for your input.
  
  Best wishes Graham