To be fair, his use case was/is for writing very simple things (blurbs on his blog), that simply don't get into painful corner cases. It's a simple tool that works great - for simple things. It can fail hard on more strenuous cases. As you say, he resists attempts to resolve the ambiguities, yet, he also guards the name "Markdown" - which has all the mindshare - causing headaches for others who try to resolve those ambiguities. So, we get things like CommonMark.