Ugh, so I went through about a hundred documents when this story first broke, and I swear I found something about the eTVB bug including a misreport of a thermal junction temperature which permitted the overvoltage. I can't find it now, at least partially because "intel" "eTVB" "temp" search terms in Google come up with about one hojillion hits versus what it came up with two months ago. Bleh. I wish I would've just posted the link. So, for now, I suppose take it with a grain of salt? I'll keep looking.
The firmware / microcode thing is old BIOS workstation admin knowledge. Intel CPU microcode goes hand-in-hand with the Intel Management Engine firmware. If you've got Windows and a modern-ish Intel processor, you'll find the Intel Management Engine drivers are what expose the microcode update path in Windows -- Microsoft's own driver repository provides the IME drivers to make life easier, but you can trace it via the provided INF file. The IME firmware is disconnected from the BIOS in a general sense, however it can all be bundled together in the same flash file to simplify an end-user's life. If you've ever done firmware upgrades OTA via Windows, you've probably dealt with the discrete update modules of the IME and the CPU microcode.
All that to say, in my experience the motherboard firmware can usually be rolled back (modern boards are better at this than older ones) however this doesn't automagically always roll back the IME firmware, which is also linked to the CPU microcode. The overall behavior would be unique to the manufacturer of the BIOS update tool; Intel provides mechanisms to do this work as I mentioned in my prior post. If whoever wrote the BIOS upgrade (downgrade...) utility wanted to, there's no reason they couldn't write the additional logic to roll back multiple firmware images in a single command.
