Further to what I put up in the thread earlier, in a normal electronic rolling shutter sensor you couldn't do a 30 second readout easily as lines are automatically read out while others are acquiring, therefore some will have completed, while others are only just starting, and your 30 second exposure will be much longer in elapsed time - though in theory each row will only be exposed for 30 sec
In a stacked sensor, the readout of each rolling line is done very quickly into the next level of the stack, allowing the fast frame rates, this must run as a repeated process and effectively the integration of the image is taking place within this next level in the stack. When the image has been completed this stack levbel can be read out.
There must be some kind of limitation internally on how long (or how many) images can be integrated for. I don't know what this limitation is, it may be purely firmware in that Sony just don't expect someone to use the electronic rolling shutter in this way, it may be physical, we don't know how fast the readout is from the sensor into the stack, and maybe it runs very quickly and always integrates, even if its doing a 1/2sec exposure, maybe there is a heat build up issue on the sensor/stack, when its run in this mode which might cause a hot pixel type effect.
With the mechanical shutter the sensor will be exposed for the duration, and then the data read into the stack, and then onwards, ie one one stack mnovement operation.