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.