The blog post says 810 paid subscribers, and the price was $4/month so that's $3.2k/month.
I mean that's not bad, that sounds like they'd nearly made it. Maybe they'd need 2-3x that to live off (depending where they live). But if they got that far I sort of feel they might have been able to make it. Then they wouldn't need any other job, they could just live off the product. And then any additional revenue growth would be profit.
I mean they were a lot closer to it being able to sustain them than a business with e.g. $0/month revenue, or e.g. 3 users at $10/month (I've worked for a few such projects without product/market..)