FYI notarization is not required, nor was it when this was written, nor is the developer program required, for macOS software. There's pitfalls, for sure, and distribution isn't as easy, but you can in fact still self publish Mac applications to this day.
The bigger problem is that Spelunky Classic required an old version of Gamemaker, and Apple was ahead of the curve on deprecating 32-bit applications, which means it might be harder to build for modern releases.