Yes, atomic commits is not a very descriptive term. But main/master is stable is only a necessary condition, not a sufficient one.

If you squash (or mix from the beginning) several unrelated changes into a single commit, main/master would be stable.

AFAIK atomic commits means nothing can be taken away without breaking the change and nothing needs to be added to make the change work. How to express that without 2 clauses is indeed a good question.