The prophets, such as Isaiah and Zechariah, as well as King David, would give the predicted details about the incarnation of Jesus, and Moses knew the story of Abraham and Isaac, how God told Abraham to kill his innocent son as an act of obedience, which presaged what the Father would do to His Son, willingly accepted by the Son, just like Issac seems to have accepted his apparent fate at the hand of his father, so the concept of the sacrificed son, in relationship to the lamb sacrifice (which God provided in lieu of Issac), was probably quite real to the ancient Hebrews.
Could not then have Moses been talking about Abraham in that verse then?
The point is, if Isaiah, Zechariah, and King David could find it in themselves to be clear with their prophesies about Jesus, then why couldn't Moses.
One other thing. Is the "heel crush/seed enmity" verse posted above the only writing of Moses which could possibly prophesize the coming of Jesus? Surely Moses would have written more about it if he believed in a Messiah.