Hunter
For all the ones thinking that the crew should have applied brakes, I recommend a refresher course...
I don't think we fundamentally disagree, but I repeat what I said earlier
... the best way is by applying a small amount of power, but if the engines haven't been started, gentle braking is the only way.
Maybe you suggest that it is better to roll down a steep slope and / or run into another aircraft as a way of stopping the aircraft?
In this case, with the added weight of the towbarless tractor attached, there may even have been less risk of standing it on its tail using light braking.
As always, the decision must rest with the Captain if braking is to be used, to say it should
never be used is wrong, imho.