The game requires Unity to run (you can say it has a proprietary dependency). So it is proprietary. Just like Google Chrome is not Free Software just because it was built on top of Chromium (which is Free Software). If users don’t have control over what is running on their computer then it’s proprietary software.
The IDE and build engines may be proprietary. That doesn’t make the code base of this project or anything using Unity proprietary by default.
The game requires Unity to run (you can say it has a proprietary dependency). So it is proprietary. Just like Google Chrome is not Free Software just because it was built on top of Chromium (which is Free Software). If users don’t have control over what is running on their computer then it’s proprietary software.
So it’s proprietary then. OP didn’t ask for FOSS, he asked for open source. There’s an MIT license directly in the Github for the project I linked.
What is the difference between FOSS and open source?