I would like to use the Si8751 as a high side FET driver like in Figure 3.1 of the datasheet. Table 4.8 of the datasheet says that the voltage on any pin can't be higher than VDD +0.5V with respect to ground. Does this mean that the source pin (Pin 5) can't be more than VDD +0.5V with respect to the ground pin (Pin 4)? That is the way I read the datasheet but it doesn't make sense if the input and output sides of the chip are supposed to be isolated.
You are correct that this phrasing seems illogical. It should read something like, "Voltage on any pin, 1 through 4, with respect to ground". However, even that doesn't account for the Si8752 which doesn't really have a ground pin.
You should have no problems using the Si8751 as a high side switch as shown in the diagram.