There are a number of ways to dump out of autos on the 737. I don’t know how different the auto throttles on the 777 are. The initial reports looked like he clicked in and out of auto throttles several times on the final approach.
If the B-777 autothrottle is used during a manual landing, thrust is automatically reduced to idle at 25’ radio altitude if the pitch mode is “vertical speed,” “Glide slope” or any Vertical nav mode. This could have happened while over the water if they were that low. (Conjecture)
Most likely the A/Ts were OFF.
If the A/T switches are off, they prevent any autothrottle engagement. These switches are on the mode control panel just below the glare shield, visible and reachable by both pilots. To “click in,” these switches must be used.
My hunch is that the pilots in this case, Asiana-(SFO), were as weak in system knowledge as they were in visual approaches.
The autoflight systems on the 777 are complex and any weaknesses, on the part of the pilots, are magnified and can result in a disaster. Add cultural problems such as being reluctant to cause another to “lose face” if corrective suggestions are made and the recipe is complete.