Why does the foreword say this story might be a parable?

The Parable of the Pearl (also called the Pearl of Great Price) is a parable of Jesus of Nazareth. It appears in only one of the Canonical gospels of the New Testament. According to Matthew 13:45-46 the parable illustrates the great value of the Kingdom of Heaven.

It immediately follows the Parable of the Hidden Treasure, which has a similar theme and the parable has been depicted by artists such as Domenico Fetti. A version of this parable also appears in the non canonical Gospel of Thomas 76.[1]