Most of this writeup is not about Markdown, per se. It's about interoperability issues with extensions to the base syntax that are not implemented consistently in all renderers. The exception is blockquote formatting.
These include: LaTeX math (not Markdown), mermaid diagrams (not Markdown), github callouts (not Markdown).
The rest is a call to action to use a paid service they developed. It's an ad.
Fix MD is quite a handy feature, especially when you have raw Markdown generated who-knows-how. I’ve used it myself multiple times with Mermaid and weird LaTeX notations