The author mentions bike repair as a hobby, and the issue with trying to turn that into a monetized hobby will quickly become apparent once you talk to some experienced full-time bike mechanics. As a hobbyist your scope is almost always rather narrow, i.e. you may know all about how to repair your particular model of bicycle, and perhaps a few others. In contrast a professional will deal with dozens of models and will also know who to contact for specialty jobs (some of the early suspension bicycles, for example, require highly specialized tools that only a few mechanics own and which are no longer produced).

It is sometimes possible that a hobbyist can find an entry point to the professional world. Building wheels is something I've recently been learning, but there are a lot of people who've been doing that for over a decade and they can probably build a wheel in 1/10th the time it would take me. Given the supply chain backups and the scarcity of bike parts today, however, there might be a little niche there.

Point being, the leap from hobbyist to professional in any area can be a lot larger than it appears at first glance.