"Hard drugs are plentiful at this level" appears to be something the author invented out of thin air. Cigarettes, alcohol, marijuana, and "hard drugs" all cost money and therefore the rates of abuse of these things are positively correlated with income, which should not surprise you.

There seems to be evidence for the OP's thesis.

E.g., "Unfortunately, one’s economic status, especially if they are impoverished, can increase their likelihood of experiencing a substance use disorder, as this is one of the most common risk factors for this condition." (https://adcare.com/addiction-demographics/socioeconomic-grou...)

These things would be positively correlated with income only to some bare minimum level. Most drugs seem to be relatively cheap, so it doesn't strike me as odd that someone with an utterly miserable life would be more likely to use their meager savings towards substance abuse to try and temporarily make themselves feel better.