Jᴀᴄᴜʀᴜᴛᴜ - Tʜᴇ Cᴀsᴛ Oᴜᴛ

And who else would believe shit like that, if the smugglers tried to tell anyone? Most people probably don't even think a worm is big enough to be ridden if they haven't seen one. Them crazy fremen.

And if the smugglers, of all people the most likely to find out about such a thing, didn't know and only had rumours, it's safe to assume that no-one else knew.