There are a number of options that can ease this, as the onus should not be on the user to feel they have to go and drop it in its original position - assuming they can even remember where that might have been.
One is to provide an undo redo capability. This puts users much more at ease in that they know even if they do drop something somewhere where they don't want then they can just undo it right away.
It doesn't have to be a long and complicated undo history - even a simple 'Undo last action' would put users at ease.
Change cursor outside drop area
Another option is to change the cursor when the user moves the mouse somewhere that they can't normally drop.
Typically this is a 'forbidden' style of cursor, but that kind of puts people off dropping outside at all, so you could experiment with different cursors that indicate better that you can let go of the mouse and nothing will happen Something that indicates 'cancel drag'.
In many mainstream applications, users will be used to being able to press escape to cancel the drag operation.
You should provide that as a way out for users familiar with that. I notice your linked example does not allow escaping of the drop.
Drop safe zone
For users less familiar with drag and drop or who may not discover the undo mechanism, then when the drag operation starts you can show an area at one edge of the region which you can label 'Drop here to cancel the drag'.
This is a great 'just in time' mechanism as it's only displayed when the user has actually started a drag.
It makes it clear to the user without then even having to explore what happens when they go outside the droppable area, although in essence that is exactly what they are doing when they drop in your 'safe-zone'.
Use the original item space as the drop safe zone
An alternative to showing a special drop safe zone in a common location is to leave the space where the original item was dragged from rather than dynamically removing it and shuffling the tree. Typically a dotted line around the source position is enough and you could still label it 'Drop back here to cancel the drag'.
This does have the down side that the tree may have scrolled the original item out of view during the drag.