But most indie games do not roll their own engine. Even this widely shared post aiming to show off games with custom engines, ends up listing a ton of games with notes like "OGRE-based" or "MonoGame-based." I'm sorry, but that's not a custom engine. That's an atypical engine choice (OGRE or MonoGame) with tweaks. Maybe I'm being a pedant, but I think the difference matters.
https://gist.github.com/raysan5/909dc6cf33ed40223eb0dfe625c0...
Unfortunately I don't think there are good numbers on how many truly original / custom / no-engine games are out there, in absolute or market terms. 13% is the ceiling, but I wouldn't be surprised if it was much smaller (e.g. <5%).