Not speaking for OP but Steam has some interactive stats [1] Where as of May 22 shows Macs to be ~2% and Linux 1% of install base.

[1] https://store.steampowered.com/hwsurvey/Steam-Hardware-Softw...

Author mentioned that they only use unity for the rendering and audio, cross-platform games will use unity for a lot more, and generally get cross-platform for a significantly reduced "cost".

This project is in C# so that's the first hurdle, I'd guess.

C# has had cross platform support for 17 years. Without getting into the horror show of absolutely counter-intuitive acronyms that is modern .NET development (of which C# is a part), .NET now directly and natively supports cross platform targeting. I can build a Linux/Mac/etc binary from my Windows machine with the same compiler by changing a single flag. It's pretty cool stuff.

The Uno platform is a third party project that ties this, and more, together to enable building single code-base cross platform apps with native UIs simultaneously targeting Windows/iOS/Android/Web (WebAssembly)/Linux/macOS/Surface Duo.

Compiling modern C# is usually not a problem on Unix. Building the relevant tools yourself is a challenge, but the average macOS dev won't be doing that.

Perhaps using Steam's hardware survey:

https://store.steampowered.com/hwsurvey/