This wouldn't be the first time a person that can't use nen uses nen unknowingly or even have a specific ability. When Gon and Kiluha raised money to buy the Greed Island game they discovered that some of those artist used nen unknowingly despite never being trained in it. Also that fortune telling mafia daughter. She even had a own ability that told the future, even though she couldn't use nen herself. Is it really hard to believe Alluka could be the same? Isn't telling the future also out of this world crazy magic? Maybe not as much as Alluka but still.