Forgive my ignorance, but is it not required/necessary to have one hand placed on the throttle levers during a critical phase of flight?
Yes but as during a normal takeoff at the V1 call or Vr if the two are coincidental both hands are placed on the control column. So it is not unusual to take one's hands of the thrust levers during a go-around once the TOGA switches have been pressed and you have verified that the required power is being provided or, as in the case of a rejected landing after touchdown, once the thrust levers have been manually positioned to provide sufficient power.
On the wider subject of training and standards I have been involved in some training for Emirates of experienced, but new to Emirates, co-pilots. Without exception those who I had any dealings with were all very competent, diligent and keen. Emirates training is also comprehensive, thorough and the standards set are high.
With an airline of their size and with the number of pilots employed it is inevitable, as with any airline, that someone somewhere will make a mistake. In this case the mistake really should not have happened but that can be said of many aircraft accidents.
The landing was deep but in Emirates a 'Long Landing' warning from the RAAS is a compulsory go-around. Without the RAAS system the landing would probably have continued, there was ample distance to stop easily.