What ultimately got the thing working for me was right clicking on the executable > Show Package Contents > Contents > MacOS > Mac_Runner.

Since then double-clicking the app seems to work ok without the above workaround, but exporting doesn't work (lol). Thus the above way is also the only way when I hit "export" do the .png or .gif sprite sheets actually save. Perhaps a weird permissions thing I don't understand?

Anyway, hope this helps folks!