I think the author underestimates how amateurish many people selling keyboard parts are (i.e. rarely fully or never committed to the work as their employment, and without any experience in sales, manufacturing, marketing, distribution). Optimizing pricing or marketing strategies is not, afaict, deeply considered. Money is consistently left on the table. Look into the madness of first-party 'artisan' keycap sales, for example.
To be fair, there's nothing really wrong with this, since it is just a hobby for most people involved, and these sorts of products really often are just making something you want to exist and selling a few more to meet minimum order quantity.
As you say, this is not a massive commodity market. A lot of the folks, if they've been involved for a while, know each other. People gift each other things all the time.
It is a market, but a lot of the participants have motivations other than profit-maximization.
To be fair, there's nothing really wrong with this, since it is just a hobby for most people involved, and these sorts of products really often are just making something you want to exist and selling a few more to meet minimum order quantity.