Well, the only other explanation would be that she was just plain wrong, which would be even shittier psychic powers.
My serious hope is that Luffy hears them talking and goes "Oh, your mom wanted everyone to leave the island but they're too afraid? Okay." *destroys island*
That kind of thing would be right in line with his encounter with Laboon. Were he does something that makes him seem terrible because they'll be better off for it.