This is a great question that opens the discussion of what the nature of scripture actually is. Your priest sounds like a thoughtful and well-educated man and he's pushing you to think more critically about this biblical passage.
What you are describing is based off of the work The Parables of Jesus by Joachim Jeremias. The Basic premise is that the Gospel parables, in this case the parable of the sower, represent different "layers" of tradition. The theory goes that the parable itself likely is an authentic saying of Jesus—which was left open to interpretation (cf. "He who has ears, let him hear") However, when the Gospels were written (here probably the parallel in Mark 4) an additional "layer" was added to help explain the parable to those hearing the gospel read aloud. In this case the "explanation" is labeled the "Early Church" layer, because the it represents how the early church interpreted this particular saying. Of course the Matthew passage is basically the same, Mark was probably just written first, but that doesn't mean both authors had to use the story to make the same rhetorical points in their gospels.
The issue of authorial intent and its role in determining church doctrine is a touchy issue. However, since you ask for a Catholic answer, I would hasten to add that this fits well within the Catholic tradition—that is, Catholic dogma has always been and continues to be an active and living tradition informed by the scriptures, but not to the exclusion of reason, nor previous tradition.
The Bible is just the very earliest part of this traditioning process.
N.B. I'm not Catholic.
