I think of sobriety as relating to alcohol only, and the online definition I just looked up only uses it to relate to alcohol or alcoholism.
But you could be alcohol free and still get high on crack or some other substance just as bad or worse than alcohol.
I don't know much about weed but I think it's possible a person might be affected horribly by alcohol and not marijuana. Or the other way around.
It might depend on the person. Do you think you are in danger of abusing it? It's mostly your call to make IMO, but to answer your question, yes I'd say anyone who's alcohol free is sober.