Hello. So I did some tests in 3 machines: a Big Sur 11.5, a Mojave 10.14 and a Windows
On Big Sur, there is a grey stop sign to the left of its name in Library. However, It doesn't mean that it cannot be downloaded but things happen this way. I have tried download some other STOP-signed games, it works alright.
On Mojave, this is NO stop sign (it's before the Catalina 10.15 update), and it refuses to download too (0 bytes downloaded, complete instantly, while starting it says cannot find the application).
On Windows, work perfectly.
So I assume that the steam on mac cannot access to the application somehow.