They’re entirely different mechanisms. Electronic front curtains work entirely differently than a fully electronic shutter. I’m not sure how much you understand about shutters and their mechanics, or the way camera sensors work, so I’ll try to take a middle ground explanation here…
An electronic shutter blanks every “pixel” in the sensor, and the iterates over the pixels and reads their values. The blanking signal can easily be synced, and is effectively instantaneous. Iterating over the pixels takes time - 48MP at 10bits is a lot of fucking data, and it is extremely complicated for the sensor to be able to read out that many pixels, the memory buses to transfer the data, the processing units to process the data, and the storage mechanism to write that data. It’s not a long time, but it takes fractions of a second, so it approaches shutter speeds. This means the whole mechanism runs “line by line” so the later lines are read “later in time” from the time you clicked the shutter button than the earlier lines. Even if you sync the blanking signal to match this later read time, in order to keep the “exposure time” constant across pixels, the pixels are still read in order and the later read pixels are capturing a later point in time than the earlier read pixels. We just can’t read from an entire sensor within 1/1000th of a second.
That being said, electronic front curtains are way simpler. You blank everything together, then you snap the rear curtain closed, then you read the pixels. Even if the later pixels are read later, they are behind the rear curtain, so they aren’t exposed to any of the “later light” - the exposure data in all the pixels are “saved” from the same point in time. Even though they’re read later, the rear curtain ensures they never received light beyond a specific point in time. As long as you read the data after that point, there’s no noticeable variance in the point in time that the pixels were exposed to the picture.
Again, this is a bit of a simplification - there is a lot that goes on in the syncing process between curtains to be able to obtain really fast shutter speeds, but the concept is essentially as above - a fully electronic curtain is constrained by the read time of the sensor, but even an electronic front and mechanical rear shutter can use the light-blocking properties of the physical shutter to ensure better synchronicity between the different sensor lines.