The surface meaning is that Decoud is safe, because Nostromo is unlikely to betray his position on the island to the Monterists. But the deeper meaning is that Nostromo is "safe" from being crushed by the sense of solitude afflicting Decoud, because he has built his egoism into the dream-ideal that sustains him. Note that only after coming to this conclusion does Decoud put the grapnel on board, sending Nostromo alone into the gulf of darkness.