I did find the following line interesting.
" Stage theory posits that there is an invariant sequence in initiation and use of drugs, with use of cannabis preceding the use of "hard" drugs such as cocaine and heroin.8-10 This theory has been highly influential in drug policy debates and has provided a major rationale for sustaining prohibition against cannabis,11 as it is assumed that delaying or preventing early cannabis use may reduce risks of other illicit drug use.
So in the one part they are telling you that it is imperative that the gateway theory be validated as that is a key component in supporting the whole prohibition against marijuana, they then go on to tell you in the next part of the sentence that the whole gateway theory is assumed.
If I read it correctly, they did identify some individuals who had used some other classes of drugs prior to trying marijuana, and excluded them from the test. They don't seem to have made any attempt to determine if alcohol was used prior to trying marijuana. Curiously, they consider the tendency for an individual to become alcohol dependent after early exposure to marijuana to be of significance.