As far as I can tell (as not a game developer) from the introduction videos is that the game porting toolkit / Wine is designed to shorten the time it takes to

As far as I can tell (as not a game developer) from the introduction videos is that the game porting toolkit / Wine is designed to shorten the time it takes to get a first macOS version of your game running and to evaluate how well does it run. A (near) zero-effort rough draft of your potential macOS port. They show that previously you'd have to take the Windows version, port the source code, port the HLSL shaders, implement graphics/audio/input/HDR and then you'd be able to get a first running version of your game that you could evaluate. And now with the Wine tool, you'd take the Windows version, run it through the Wine tool and then you'd have the first running version version of the game to evaluate. And you can then evaluate what parts of the games run good and which don't, is it utilising the GPU properly, and so on. And if you decide that there's potential in the macOS version, you'd then start doing all of the stuff that was previously required to get the first running version.