gen_init_cpio is not the original cpio archiver utility. It's only obvious to know to use it if you already know about it. The Linux kernel docs cover it, sure,

gen_init_cpio is not the original cpio archiver utility. It's only obvious to know to use it if you already know about it. The Linux kernel docs cover it, sure, but the whole point of the OP is a more approachable introduction to the same information.