I'm not sure what the legal definition of hijacking is, but here's a dictionary one:
illegally seize (an aircraft, ship, or vehicle) while in transit and force it to go to a different destination or use it for one's own purposes.
The key words here are "illegally" and "seize". I don't see how he "seized" control, as the captain left the FD voluntarily and left the FO at the controls. Secondly, there was nothing illegal in so doing, by either the captain or the FO. This only became an illegal act after the FO refused the captain re-entry to the flight deck, but I don't think it can feasibly constitute a hijacking (at least not in the conventional sense).