I think you're underestimating the Yoga explosion that has been happening recent few years. It's insane and certainly becoming very mainstream.

The yoga community has been growing by 30% every 4 years for more than a decade.

I'm confused. Are you saying that Git is more or less mainstream than Yoga? If more, then woa you're filter bubbled.

To be fair to them, they state they have never received funding and the site is profitable since the end of April. But it's very niche alright.

but the product and target market would have been known at the time of initial application - so to invite them to in-person interview suggests this niche and alternative product was suitable. This may give weight to some of the more "murky" reasons for rejection the author alludes to.