If this had occurred within Chinese waters, I might be inclined to agree with you. However, this occurred in International waters, and your government had no right to steal the vehicle.
So in the legal opinion of all the world minus China it is theft. In the legal opinion of China it is not.
Unfortunately this subtly can not be expressed in a 140 char tweet so the situation deteriorates weekly.
(EDIT: beautiful details by jwtadvice here ) https://news.ycombinator.com/item?id=13195427