itch.io

Ah, here's the new comment explaining the whole process in case you didn't see it yet!

Did you use a zip file when following the tutorial (for example, right-clicking on a zip file and then clicking Get Info)?

If that doesn't work, or you don't have any zip files on hand, you can instead manually extract the zip file with Unarchiver without setting it as your default, it just means a couple more steps depending on the browser you use.

For Safari and I think Google Chrome also, it will automatically unzip the file with the default app (which probably is Archive Utility). You can stop this process by right-clicking wherever the download process is being tracked on your browser, and then clicking Stop. Make sure to do this after the actual download is complete, so as soon as it says anything along the lines of "decompressing the file". If done properly, you should be left with just the zip file of the game, which you then can open in Finder. Right-click it, click Open With, and then choose Unarchiver. 

If you use Firefox however, you can choose from the start which application to open the zip file with, so you can just skip all these steps and just download the game normally, but this time choosing Unarchiver instead of Archive Utility.

Another workaround to this problem that might be easier is to use Unarchiver to extract the game's zip file instead.

1. First download Unarchiver, which is free on the App Store
2. Set Unarchiver as your default for opening zip  files, which you can do by following this tutorial.
3. Download the MacOS file for the game
4. If it has worked properly, when opening the game you should get an error about MacOS being unable to verify the developer, which you can get around by two-finger clicking/right clicking on the app and then clicking  Open and then Open again from the following warning dialog.

This method doesn't seem to work for Safari for some reason, but there's a workaround for it I can explain in a reply if anyone needs it.

I can second this claim! I don't really know how it works, but using Unarchiver to extract the zip file seems to be a more easier solution to the problem than the one with the Terminal. This also worked with one other game that initially wouldn't open on Mac Catalina for me. Also, if anyone needs more help or steps with this method, I can make a new comment explaining the whole process over.