Make this procedure more robust & sustainable to prevent a recurrence. That is what is needed, not a blame game, when peoples lives are clearly at risk.
Well now's the time to propose sensible solutions. Input from pilots, heavies down to commuters, plus the guys who drive the de-icers, could well produce an international standard. The old railway token system is a possibility (but practically - unlikely). Example - If any de-icer working an aircraft had their token in hand then flashing red lights would show ahead of the pilot. The lights would only change to green when all vehicles were clear, confirmed by the drivers plugging in their tokens to an 'off area' token plinth. As de-iced aircraft moves on tokens are removed from plinths and lights change to red for next aircraft to pull forward - to lights.
Needs some refinement - but ........ any other ideas. Only simple ideas that aren't affected by blocked radio transmissions etc.