The reason I decided to opt-out of caffeine is that I thought was abusing it to force myself to do work I didn’t actually want to do. I also had the hypothesis that the human body should be fine without it, as we’ve had most of our biological evolution without it. A wolf doesn’t drink an espresso before chasing a deer, after all.

This excerpt made me stop taking this seriously. Using caffeine or any other stimulant for the purposes of doing work one isn't comfortable with doing is a character flaw (or an act of desperation, however mild), not a fault of the substance one chooses to lean on. Justifying abstinence from such substances on grounds that the substance isn't intrinsic to human biochemistry is both a naturalistic fallacy and a bizarre rejection of human social history of tool use (substances are just another tool).