[icandoitbetter]

    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.

[d0mine]

You might mean: a median niche business makes more money than a median startup.

Average is much higher than a median for startups because it includes the most successful startups which are by definition have a high growth rate over an extended period of time so they are very big.

Niche businesses in this context have a low growth so their average is closer to the median. The growth is constrained at the top in absolute terms (otherwise it would be a startup) so the average is lower than the one for startups.