Also from my reading of that, the T2 chip doesn't reboot with the laptop -- it stays powered on at all times after it first starts up. If that's really the case, a reboot won't do much except kill non-persistent things that are running on macOS itself.
So, a) a T2 hack persists until such reinstall and b) you don’t need a persistent beachhead if you’ve taken the hill and maintain persistence there. This exploit opens the door to that, and you can install keyboard intercepts via the new System Extensions Interface. Did it yesterday myself via an update to Karibiner. Notarization can be disabled by interrupting the network pathway and you can definitely disable SIP after you capture the password. In essence, T2 is the lynchpin that prevents the defeat of other aspects of macOS security.
I don’t think that’s 100% true. Yes, you need persistent hardware to re-root the T2 on every reboot. But what about a one and done T2 attack - a drive by keylogger installer? Get credentials and then mount a more traditional attack to install APT/RAT.