No, it’s saying it works better than INSS, without resorting to GNSS.

So, like terrain-matching, but gravity field. Cool!

Not so much dumb as suspicious.

The framing device for the whole movie is that they’re searching the wreck to find a priceless necklace, and bring along an old woman, who had it when the ship sank, to help them. She then tells them her story, moves them into abandoning the search, and in the end it’s revealed she had it the whole time when she throws it overboard.