I think the author has made a number of errors in there post and seems to gloss over the numerous social problems that arise from alcohol consumption. Th reason Alcohol is consumed by a significant portion of the population is because its a drug that society doesn't perceive as a drug. It carries all the negative physiological and social effects other drugs have but it doesn't carry the negative association of being what we consider 'a drug'.

Yes It's a form of social lubrication and it might be pleasant to consume but its efficacy as a form of verifying trustworthiness is tenuous as best. Most high functioning alcoholics would be able to maintain an air of trustworthiness which is why alcohol itself can be such a destructive force.

It's interesting that alcohol is explicitly banned in Islam You could use the approximately 1.1 billion muslim(probably closer to 500-800 million practicising) population as a control abstaining group and look at whether communities that consume alcohol have a higher propensity to trust each other or not.

Edit: At the end of the day I think regardless of any philosophical debates the simplest answer might be that people consume alcohol because being drunk feels good.