Hmm, it look like this is a new security feature with Big Sur and not something with the game itself. Unfortunately, I don't have a recent macOS device to test with, so I can't verify a fix myself.
It looks like there may be some commands you could try in Terminal to fix permissions and allow the game to run. First off, this video might be a good starting point:
You could also try this command before or after the steps in the video (replace "/Applications/AppName.app" with the path to the game):
codesign --force --deep --sign - /Applications/AppName.app
Let me know if this helps!