That's not what they're doing though. There literally going to drag a magnet across the sea floor and examine anything that sticks to it. Which will be largely iron, nickel, and cobalt. Were it to be alien tech, it probably won't have large amounts of those and won't stick to the magnet. At least it seems that way to me. Could you make a magnet that would create a strong enough attraction to pick up a piece of gold? I suspect not, or there would be a reality TV show with rednecks trawling the ocean in Alaska with magnets, searching for gold.