So the SSPX knows what the Faith is. They have catalogued its contents in detail in the letter. They have stated that denying even a single point of the Faith destroys that Faith and renders Communion with the Church radically impossible. The man they address, Bob Prevost, denies multiple points of the Catholic Faith. They call him their Holy Father and ask him to confirm them in the Faith that he does not hold.
Therefore, the SSPX will not say what their own premises demand: that Prevost is not Catholic and therefore not the Holy Father and therefore not someone who should be asked for confirmation in the Faith. The antipope and heretic should be anathematized as stated in Galatians 1:8-9 — "But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be anathema."
Any so called Catholic of good will its time to support faith of our Fathers express in this concise and precise statement. Viva Christo Rey