Sure, depending on how you interpret initial development. FastAPI has 60k stars on GitHub, has been extremely popular for at least four years, and is widely used in production by a lot of people. It's the maintainers' decision what their goals are for a v1 release, but I was personally surprised to learn that it hasn't had one yet. I can see why one might argue that they're not following the spirit of semver at this point.
As long as the developers/maintainers feel like "Anything MAY change at any time" and "The public API SHOULD NOT be considered stable" is true, both the "spirit" and specification (if we may call it so) say it should be on 0.x.z
