When the meaning of a world is largely accepted in spite of its vague character, it is a sign that its introduction made it so acceptable. And this is more saliently the case of ‘soul’.
That or those who introduced the meaning of ‘soul’ knew that they enter in an area of human expectations for self-understanding, where even a vague word as ‘soul’ helps.
But even if this world benefits of wide acceptance, it cannot be overlooked that this is a word that has its origins elsewhere than in the range of evident things embraced by the supposed possessors of a ‘soul’.
Therefore, self-knowledge cannot be identical with the knowledge of soul.