You have to launch the .app file if you want to play the Mac build.
I've heard from other developers that macs don't like to open Itchio downloads which aren't certified as a security feature, so your mac might refuse to open the file. If you can't get the .app to run after downloading the game from Itchio, you would be better off claiming a free Steam key for the game and launching it through Steam instead. Steam's launcher can bypass Mac's security and the game should open properly then.