Something fun mentioned in a comment on that article, this the majority of this "pre-boot" code is actually FOSS in POWER9. There are a set of "auxiliary processors" called "PPE"s, among which there is one, the "SBE", or "self boot engine", which is a very small and simple PowerPC core that IPLs the big POWER9 cores [0]. These big processors with tons of cache and interconnects need a lot of help to get to executing PC 0x00.

I suspect that almost all the big "application processors" from Intel and AMD, and the exotic ARM/SPARC server chips, have equivalent embedded ICs to jump-start the "big cores".

[0] https://github.com/open-power/sbe/blob/master/src/sbefw/app/...