I think the parent's point was meant to illustrate that there is very little boilerplate necessary beyond the CPU emulation (which is, arguably, the fun bit).

On more modern systems, the CPU is only a tiny bit of the equation. You might have to emulate the graphics pipeline, the audio processor, the input mechanism, etc... This makes the process of writing an emulator more tenuous, as those are not nearly as straightforward to implement, and might be badly documented.

If you look at the really modern platforms (like the Switch or 3DS), CPU emulation is barely 20% of the work.

You've got it ... even the Atari 2600 has a graphics pipeline almost as complex as the CPU. The Space Invaders/Midway 8080 platform is rudimentary by comparison, maybe owing to its pedigree as the first arcade game platform (Midway's "Gun Fight" in 1975 has virtually identical hardware)