...and Andrew/bunnie doesn't answer them, I can. I'm very briefly mentioned in the book under a different Internet name that I'd rather not say here.
I was the person who figured out how to dump the second version of the MCPX's secret boot ROM without having to repeat the HyperTransport bus tap craziness that Andrew did in the first place. Namely, the A20M# attack, which was much easier to do. (If Andrew hadn't done his original attack, though, we wouldn't have had the knowledge necessary to pull off my attack. <3 Andrew)
We kept the A20M# attack secret until the 360 was released, in case another MCPX silicon revision was released. It turned out that Microsoft had, in fact, coded a new MCPX ROM to defeat many of the exploits used to hack Xboxes - they just never released it, probably because it would've cost a fortune for what was then a console in its late stages. We didn't find out about this MCPX ROM update until some people looked into how the Chihiro arcade boards worked in 2014, which showed the new MCPX code in the debug ROMs. The A20M# attack still would have worked on this design - it was an attack on entire secret boot ROM design, not the MCPX ROM's code =)