If it makes you feel any better, the Markdown part is optional (and has no semantics). Somehow it feels about right that the Markdown file can actually just be a YAML file with the wrong extension.

(Actually, to be more specific, a YAML file with no directives, explicitly-signalled start-of-document-content, and followed by a second null document. I will note that frontmatter syntax is not specified; the non-normative Appendix B is the only place that suggests it means prefix and suffix --- lines. And no, frontmatter is not part of Markdown, or CommonMark, and is in fact incompatible with both. And it’s invalid YAML too, the end-of-frontmatter line should be ... to indicate end of document without starting a new document.)