If the current spec for CommonMark is still were considered final 1.0, then I agree. But Sematnic Versioning has a key provision for “pre-release” strings that I suggest would be essential to successfully applying semantic versions to a community-driven project.
The spec seems still to be in a state of flux, so I’m not sure who/when/how a final Version 1 version gets (or has been) declared to use as the baseline for considering what constitutes a breaking change.
I think getting @jgm to adopt a pre-release string for ongoing edits would be essential to making semantic versioning work for the spec. In addition, considerations still under discussion on the normative vs. informative topic are essential to prevent semantic versioning runaway as @Zengat described.
Its a topic worthy of the attention of the core contributors, and I suggest it belongs in the Spec category rather than Implementation.