The Space Invaders (Midway 8080) platform described here is really easy to emulate; it's just a 1-bit frame buffer hooked up to a CPU. Subtracting the CPU emulator, my implementation on http://8bitworkshop.com/ takes less than 200 lines of code. One of the easiest with which to get started.
I also started out on the Apple ][, writing my own versions of the games I saw in the arcades (donkey kong, pacman, joust, etc).
My kids are old enough to be curious about how games are actually built, and we've had fun building an adventure game in basic, and a version of asteroids in C. We had planned to build something this summer, so this book of yours is an ideal next step which we can work through. I think they'll really get a kick out of seeing how it all goes together. (okay, okay, I admit it. Nostalgia hit me hard and I just want an excuse to fire up my old Apple ][ again.) Regardless, I just hit "order" on Amazon.
Similar with a system I put together for emulating CP/M recently. Take away the Z80 emulator and you really only need to emulate a few bits of hardware for handling console I/O, selecting disk drive, track and sector and then reading or writing the sector. At that point you can run simple CP/M commands, assemblers, C compilers, BASIC interpreters etc.
Add VT100/ANSI terminal emulation and you can run Wordstar, Rogue, dBase 2 etc.
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)
