If someone answers "No" to the question of "Is a pot-smoking JW really a JW ...?", then that person is committing what is called the "No true Scotsman" fallacy.
I know what you are saying, I think it's based on a fear of the example you give of someone being abusive. You don't want to let the WTS off the hook by saying "well they are not really a JW!"
But I think those are two separate things. One is whether people truly believe and follow the teachings, trying their best to live up to it. And the other is people who happen to self-identify as being a member possibly because it provides them some cover and access to people, but who's actions show they can't possibly really believe it.
Only the first is really a JW, but the latter is still the WT's responsibility for providing that environment and not policing it well enough.
Obviously smoking pot is nowhere near the same severity of crime in secular terms, and like most things in the WTS, how serious of a crime it is will depend to some extent on the individuals involved and the local congregation. But I'm not sure it's something many people who genuinely feel that they are trying to be JWs will be doing.
So I still believe that the proportion of people who smoke pot will be higher among exJWs, who know then don't want to be a JW, than those who think they do.