I tend to be a bit tongue in cheek, but for more background he started in the 1970s, only a few years after stimulants were researched for ADHD treatment in children. He undoubtedly had connections in that area (the Erdós number is a thing, after all), and in the prior 30 years was known for drinking copious amounts of coffee, so better stimulants weren't exactly a difficult jump to make regardless, but it wasn't necessarily that he knew he had the condition now known as ADHD, just that they were what worked best for him.