There's no hard dependency at all. The markdown will parse just fine without mermaid.js, it basically just takes a code block and runs it through the mermaid.js code if it matches the language
The real criticism of the original comment is that markdown has never really been "portable"; yes there's now a hard dependency (for some definition of "hard") on mermaidjs for GitHub-flavored-markdown, but that actually brings it in line with other implementations. So has portability been obviated or just slightly changed?