IF this story is true, surely as someone already pointed out, receiving a clearance from ATC does NOT mean clear to taxi on its own.
Most places you can get say a push back clearance from ATC, given to you because ATC are happy there is no conflict with any other Aircraft, they do not care that you may still have the aerobridge in to the Aircraft, all sorts of equipment around the Aircraft and an Engineer still removing tie down straps etc etc.....
I do know this was not a push back situation, but the same applies, ATC were happy for the Aircraft to move, but what about everyone else.