Yeah, it sounds like there just legitimately are 50K people who are in this niche. Maybe the fact that people might assume there are fewer is why there was a gap in the market for the author to fill!
I think this is the Big Thing when it comes to making things - underestimating the market. Mechanical keyboards is a multi-billion industry, and building them yourself is a percentage of that.
That said, I wouldn't be surprised if this product managed to end up in the supply chain for a lot of the keyboard manufacturers, which would be a huge boost to sales volumes.