We can't prove that he wasn't killed by space aliens. On the other hand, the explanation is on its face quite plausible, and the level of evidence which would need to have been fabricated to support another theory is far beyond the plausible benefit to anyone in misleading the public as to his cause of death.

Drug and alcohol use, even to "dangerous" levels, is probably 20-30% of the high-profile people in the hacker scene (or, really, people in the tech scene in SFBA in general), at the very least. If Barnaby were a known teetotaler, one of the hundreds of people who knew him pretty well would have stepped forward. Given that they haven't, you can assess with high confidence that this is what it looks like, rather than a grand conspiracy.