You have proof of this?
Why do you deny it? What proof do you have against it? If you ask me for intercessory prayer (which I would gladly give), is it efficacious and why? It is efficacious, because we are part of the Body of Christ. Is the Mother of Jesus Christ part of the Body of Christ? Clearly.
May we pray to any part of the Triune God, without intercession? Of course! The Our Father is one such prayer.
However, we do ask others, whether souls on Earth or souls in Heaven, as we are all part of both the Mystical Body of Christ, and the Communion of Saints (Ephesians 2: 19-21).