How in the world did someone find this? The fact that things like this are found is a really an interesting revelation about the collective productivity of the humans race on the planet - all pushing the boundaries of knowledge in everything that we know. There is a scientist in the basement somewhere spending his/her whole life on researching a very small part of the world and maybe it will result in a spectacular finding. Go human race.
> How in the world did someone find this?
These people are Computing Archeologists - I don't know if that is a formal category but that is how I think of them. They go deeper into software and hardware of the past and bring back such gems before those are lost forever to the tides of e-waste.
Poring over the raw files, basically, looking for patterns / strings / etc that might be interesting; I'd argue that it's a bit easier for older operating systems as the modern ones are much, much bigger (MacOS 8 which was on the PowerMac G3 used in the article was 120 MB, MacOS 11 requires something like 35 GB.
But I suppose also there's less fun allowed, the article mentions this easter egg was removed in 1997 when Jobs returned.
TFA gives an extensive explanation of how it was found.
Have you ever played with ResEdit? It's a wonderful tool that shows you all sorts of parts of programs and files that you'd need tools like ghidra to show you today. ResEdit came from Apple, though, and is incredibly easy to use.