All I know for CERTAIN is that a no-start and hill start assist or parking assist faults when combined are symptoms that usually seem to require a TCM replacement for a cure.
The TCM needs to shift the trans properly to get it into position for cranking (pre-start routine, occurs when the car door is opened), and it this doesn't occur properly the car won't start.
TCM is not just a computer module, it also includes TWO actuators/sensors that do the actual transmission shifting.
A "reflash" to update software in the TCM won't necessarily cure any electronic faults (though they may show at that time) and it def. won't cure any mechanical faults with the actuators/sensors.
These cars run an automated starting sequence that's only INITIATED by turning the key to start, unlike all prev. versions where the key position actually controlled the starter. This works the same whether it's a key operated system or the push button type.
I've seen multiple reports here of starting problems that were first guessed as starter or key switch issues, as you might guess on an earlier model that's directly controlled by the switch. Many times that didn't solve the issue, as the more complicated starting system was involved. Think of it as similar to the safety interlocks with prev. automatics where everything has to be in the right position before it will crank.
IF you didn't have the hill start assist warning, there might be other possibilities. That's the key factor that made me comment on the TCM as a likely cause.