Incredible that 12 million of those sales were after 2019's Silksong announcement. I thought the game had been a big success to begin with, but that kind of performance years after release is amazing. They did put it on PS+ Essential in November 2020, not sure if that's included but let's say no.
15,000,000 x 7.50 (lowest historical sale price) = $112,500,000 in revenue on the low end (compared to $225 million upper limit full price purchases). Life changing money that continued to pour in, enough to sustain development indefinitely and allow the team to work the way they want to and ensure the game is up to quality standards.
I love it - the game, the team, their story, and Schreier's article is a great short piece on their journey to release. I'll be interested to see if the game is still built with Unity but would have thought they'd allude to that (not directly of course) as rework, which does not sound like the case. If so that's refreshing that it truly was only their passion and a lack of urgency to push the game out the door that delayed things so long.