I have no idea. I do know that it can't really be 'both' however. One would not make sense if left out, too important, but the other....it's not a direct enough statement to look like a complete add-on.
But going back to the OP, you say it is a clear statement, but somehow it is not a direct enough statement to be a complete add-on. Very strange.
It is very tricky to try to reconcile the Gospels on this point by trying to guess which one was altered. It could be either, and in fact it could be both, or neither. It is possible that the reason the synoptic Gospels and the Gospel of John seem to have different theological viewpoints is because the author of John really did have a different theology than the others.
But I think this answers the question in the OP. When we are trying to understand the meaning behind Gospels written 2000 years ago, when we don't know who wrote them, when there is contradiction between them, when we don't have the originals and have to consider the possibility that they have been altered, it is understandable that there would be a failure in communication.