This line indicates that we are getting a filtered account.  Even as John wrote it, he was perceiving the events at several removes. We who read copies of his account are at an even further distance from the divine original. What we see in the text we see as through an antique mirror, dimly and indistinctly, or through a dark and warped lens. But even so, the heart of the message can still be discerned with diligence.