There was some discussion of this on the CommonMark forums shortly after the initial release of CommonMark:

https://talk.commonmark.org/t/commonmark-formal-grammar/46

See in particular these comments by maradydd:

https://talk.commonmark.org/t/commonmark-formal-grammar/46/1...

https://talk.commonmark.org/t/commonmark-formal-grammar/46/2...

https://talk.commonmark.org/t/commonmark-formal-grammar/46/2...

I'm not sure they ever came to a conclusive answer (at least on this thread).

Edit: Here is JGM himself saying he doesn't know: https://talk.commonmark.org/t/commonmark-formal-grammar/46/3...

If GitHub is normalizing comments anyways, I wonder they could have adopted a CFG.

Overall this a step in the right direction but the whole saga is a perfect microcosm of our understructure cranking out pooly-understood stuff which comes back to bite us and cannot be tamed.

Interesting. I wonder if there's a thesis in solving this problem.