There is nothing new in how the Z9's electronic shutter works... it is still the rolling readout/rolling shutter. What is new is how fast it works.
The Nikon Z9 (and Sony α1 and Canon R3) uses a new stacked CMOS design which is the next evolution in BIS (back illuminated sensor) architecture. The stacked design essentially allows for the designation of entire layers for a particular task.
And in the case of the Z9 it allows for extremely fast readout speeds, nearly entirely eliminating the rolling shutter effect (warping/banding) and allowing the shutter to sync with flash up to 1/200. It was/is primarily the inability of the rolling shutter to sync with flash, and banding under artificial lights, that required/requires the mechanical shutter to be retained.
The stacked CMOS design approaches global shutter functionality without the global shutter drawbacks (cost, reduced FPS, etc).
I cannot say if this is the exact construction of the Z9 (or α1/R3) as that hasn't been released AFAIK; but this is one example of stacked CMOS architecture (separating the memory onto it's own layer) as compared to conventional BIS.