I believe @jasonekratz is correct: there is no true standard for Markdown syntax, and MultiMarkdown is a dialect or derivative (and mostly just one person’s idea of a direction to take). Efforts to standardize Markdown have been drawn out and controversial, and Gruber seems to dislike the current efforts (c.f. Standard Markdown Becomes Common Markdown then CommonMark).
There is an IETF RFC to standardize the media type for markdown (RFC 7763), and another titled “Guidance on Markdown: Design Philosophies, Stability Strategies, and Select Registrations (RFC 7764)” that discusses a number of Markdown variants; however, these documents are not standard specifications – they are “for informational purposes”.
The Wikipedia page for Markdown has some relatively up-to-date information on the general topic and state of affairs.