Thanks for the swift reply.
I think the issue lies in that your binary to invoke the game is linked for 32 bit environments (Mach-O executable i386). Therefore, if you used XCode, the quickest way to remedy this is to select the proper target architecture in the build system. I haven't checked your Mono, Steam integration, and Unity Engine libraries, however I would strongly suggest that they are 32 bit linked as well.
Question is: do you have a recently enough OSX target build system to actually re-import and re-compile your game project for a 64 bit target architecture? I might be able to help out if you want. I don't think certification is necessary.
From where it stands, starting with MacOS Big Sur (11.x) no 32 bit applications can be invoked anymore.