Awesome, thanks!! Works for me on Catalina 10.15.7. Except at Step 8, it didn't give me "Mac_64Bit is not from an identified developer" error. Instead, it said the app was damaged and to Move to Trash.
To fix this, I used the command Dani gave: "xattr -dr com.apple.quarantine [app path]". App path is the full name of wherever you installed the game (for it, it was "/Users/MY_USERNAME/Downloads/Mac_64Bit.app"). So the full command was "xattr -dr com.apple.quarantine /Users/MY_USERNAME/Downloads/Mac_64Bit.app".
Open Terminal in the download folder and type the above command, OR select the game file in Finder, go to Finder > Services > New Terminal at Folder, then type "cd ..; xattr -dr com.apple.quarantine Mac_64Bit.app".
After doing this, I was able to open the game like in Step 8 (at this step, it opens but shows no servers). Then I followed Steps 9-11 and opened from Steam - servers now show and I can play.