There is a legitimate distinction for it. I wouldn't, for example, call the US Ambassador to Japan or a member of the Armed Forces stationed in Okinawa an immigrant. It's a distinction becomes looser it describing people who have private sector jobs or have lived in the area for a long time but are jetting between one country and another.

They fit the definition of expat; they have no intention of acquiring citizenship nor to move their family there.

If we were talking about a young couple opening a scuba dive school and their kids navigating the Vietnamese school system, sure.