Some of the switches or buttons are daisy chained into a resistor network. The pressing of switch or button are sensed by the voltage level of resistor network. It is not only a matter of how buttons are arranged. BTW, they are arranged daisy chain, not in parallel. Whether this ADC can discriminate between single or multiple buttons is a matter of voltage resolution.
Lets take a look at some voltage combinations.
It may be read on pin PA5 on microcontroller chip. Some buttons however exhibit curious behavior. We can conclude that voltages in most cases are able to discriminate by the ADC to detect any combination, with some exceptions. Curiously these pins hold its values until all buttons are released. So by the side of ADC microcontroller alone, there is no reason that may prevent to detect several buttons at once.