Just curious - How are their servers different than FB's who also modified the heck out of them?

Sorry but I have to disagree. They are custom but that kind of customization is not all that difficult. Essentially it's using same HW as everyone else in a way that supports their scale better. That's it.

Now I really don't know anything about this, but I remember quite a while ago when configuring a Linux kernel build that there was an entire section dedicated to Google's server hardware. If it's really all that normal, why would it be Google-specific/why wouldn't there be other large vendors present as well? Talking about the GOOGLE_FIRMWARE menu.

But not custom enough for Intel to remove Management Engine from their CPUs