It may not scale, but the chances of it making (any/more) money are much higher ("riches are in the niches") than a "fast startup" as it's very rare for a startup to grow fast and monetize quickly at the same time, because most people just won't buy immediately, sometimes even if it solves a problem for them.

That is the exact point of the essay. An average niche business makes more money than an average startup, but a successful startup makes many orders of magnitude more money than a successful niche business.