The Twitter thread claims that touch input/display, touch ID, secure enclave and camera are isolated on a separate ARM SoC. Since touch ID will be used for Apple Pay, and Apple Pay will be integrated into Safari, this architecture may be harder to attack from a malicious website that is trying to obtain Apple Pay credentials. Even if macOS on x86 is compromised, there is a separate input/output system on the ARM SoC for payment credentials.
That would either cut down on the number of "real" displays the built-in GPU could support (right now it supports three: two external plus the built-in one) or would require including a second GPU. This is basically the "second GPU" option with the second GPU bundled as part of a co-processor that's needed anyway for handling payments and fingerprint recognition.
I guess it should be functional without an (compatible) OS present. Within the recovery screen and f keys using boot camp? Gotta have a tiny cpu + os for those.