People in the comments as well as Julia in the article are mentioning breaking changes. I would imagine committing to a stable API and signaling this through a v1 release would show respect to the time of your users.
Herding cats and all that.
Also version numbers are kind of meaningless. “1.0” would better be saved for a big marketing push.
Have you heard The Good News about the Semantic Versioning?
The Linux kernel, for example, explicitly states that version numbers are meaningless.
In my experience, semver ends up just being more of a reason to never have a major release rather than a way to communicate breaking changes.
I don’t think Hugo uses semver anyway…
It works really well for libraries and APIs, but in my experience it doesn't work well for end-user-facing products. If you look at npm packages, a lot use semver.
So even with semver, pre major version 1 is pretty much meaningless.