The article doesn’t seem to discuss cancellation. For example, there is the convention (at least on Windows) that pressing Escape cancels the dragging. Sometimes you also want to cancel the dragging when the mouse-up happens outside of some defined area. Cancellation serves as a quicker Undo (or an “oh, I actuality didn’t mean to drag”) for the user. In any case, this means that you have to save the original state at the start of the dragging, so that it can be restored if the dragging is cancelled, even if you otherwise provide no Undo functionality.

In the case of cancellation-when-dragging-outside-an-area, there’s also un-cancellation, meaning you resume the dragging when the pointer returns to the area, after the state visually reverted to the original one while outside the area (to indicate to the user that a cancellation would happen if the mouse button is released at that point). Or put differently, the real cancellation only happens upon mouse-up, but is already visually indicated while dragging.