Honestly I see no reason to even mention markdown. Sundown really has nothing to do with it. Just state why it's an improvement over HTML in various scenarios.
Not all markdown processors implement inline HTML, which I learned to my detriment recently (the "markdown" package on npm doesn't).
Exactly. Most Markdown processors don't implement this feature because it is very unpredictable and has too many contradictory edge cases.
... which makes Markdown even _less_ like your software. Also less relevant to marketing efforts as people up-thread are saying. Some of the use-cases that you're covering with your software were never covered by Markdown, nor were they ever intended to be use-cases for Markdown. So you're not going to find people that are currently using Markdown that will need Sundown for those use-cases.