Counterpoint: I've been employed to work on liberally licensed software for many years. In my experience, the idea of "open source" exists on a spectrum.

Open source is important to different people for different reasons. For some it's about code quality or security. For others it's an escape hatch from vendor lock-in. For others it's a moral issue. To each their own, but I think the term is heavily overloaded. That genie is simply out of the bottle, for better or worse.

I think the author goes through great lengths to clarify their position. They use "open source" to communicate something that is accurate for the vast majority of people who would be interested in this software, which I would guess is mostly people interested in avoiding vendor lock-in. For those who have a reason to be concerned about the details, it's made very obvious what the restrictions are.

Also, you're assuming a ton about the author's intent here.

EDIT: Also, I think the term "source available" in this context would cause far more people confusion/lead them to a false idea of how this project is licensed than saying "open source" does.