Semantic versioning 2.0.0 clarifies this point explicitly:
The major version is 0 for unstable initial development, before the first stable API version is published:
4. Major version zero (0.y.z) is for initial development. Anything may change at any time. The public API should not be considered
It also clearly states that the first public version starts at 1.0.0:
5. Version 1.0.0 defines the public API. The way in which the version number is incremented after this release is dependent on this public API and how it changes.
The wording of clause 8 is not explicit, on purpose:
8. Major version X (X.y.z | X > 0) MUST be incremented if any backwards incompatible changes are introduced
The wording makes clear that any major version must be strictly greater than 0 and that it has to be incremented in case of major change. But the wording leaves it open to have a version 0 for which the constraint doesn't hold, because for version 0, any change can happen any time, without need for formal increments.
Semver herewith uses the logic of mathematical induction: if you prove something for an initial starting iteration, and demonstrate if the property is true at one iteration, it's true for the next, then it's true for all the iterations starting with the initial one.
These principles are not new and were already true for earlier versions (e.g. semver 1.0.0) although with some differences in paragraph numbering.