"open source" has a specific definition[0], which this project does not meet. When people say "open source", that is the definition that they are referencing. It's the reason why there's been endless discussion about "open weights" models not being "open source".

"source available"[1] is a different thing, and you're right that this project is "source available".

[0]: https://opensource.org/osd

[1]: https://en.wikipedia.org/wiki/Source-available_software