Coming after the Stuxnet experience and the recent hack of a steel mill in Germany, which forced an emergency shutdown of the furnace, with 'heavy damage', the complacent assertion that no cyber attack could cause a reactor malfunction just seems witless. Of course these reactors are susceptible to getting hacked, the main obstacle is the relative obscurity of the control systems and the reality that there are multiple different designs in service, so that a wide ranging attack is very complicated. By the same token, the diversity of targets makes the defense much more difficult, no 'one size fits all' protocol is likely to be effective.
The hope may be that hacking a nuclear plant might be seen as an act of war, so not something most states would pursue, but the proliferation of devices makes it easier to create a hard to attribute hack. There is plenty of ill will around as well, so this is likely to be just the first such attack post Stuxnet.