Sorry I'm not exactly sure what you're saying. I know very well how it works as I write a lot of demos and games (still today) for mode 13h (see https://www.pouet.net/groups.php?which=1217&order=release) and I can program the VGA DAC palette in my sleep. Were you referring to the fact that you write 8-bits to the palette registers? That's true, you do, but only 6-bits is actually used so it effectively wraps around at 64. There are 6-bits per colour component which as you pointed out is 18-bits colour depth.
Btw I was a teenager when those Denthor trainers came out and I read them all, I loved them! They taught me a lot!
