I think it's enough... there is not much to say: just a small portion of drug market use BTC as form of payment.
The main purpose of this research is to point to the fact that bitcoin is used for illegal activities. It is not important for them how much bitcoins are involved, anyway they cannot provide any evidence that bitcoins were indeed used to buy illegal stuff. What they are trying to say, in my opinion, is that bitcoin is used to buy drugs only because of the fact that no government is controlling bitcoin. Bitcoin for them is illegal by default, no matter what things you buy with it.