As per a citation on the SSPX.ORG website that they were attacking:
Salaverri, in his Sacrae Theologiae Summa (vol. I, 5th ed., Madrid, B.A.C.) distinguishes the following:
Extraordinary Infallible Papal Magisterium (no. 592 ff);
Ordinary Infallible Papal Magisterium (no. 645 ff);
Papal Magisterium that is mere authenticuм, that is, only "authentic" or "authorized" as regards the person himself, not as regards his infallibility (no. 659 ff).
So the authentic magisterium isn't infallible.
No, please read. MERELY authentic Magisterium is not infallible. Authentic Magisterium is the top level; within the Authentic Magisterium you can find infallible AND fallible teaching. Problem is that the SSPX.ORG article the Dimonds were attacking got lazy and kept referring to the MERELY authentic Magisterium as just the authentic Magisterium.
At the end, though, the argument is just over definitions and semantics; you had the Dimond brothers arguing that the infallible Magisterium was infallible (because they defined authentic Magisterium as = the infallible Magisterium) and the SSPX arguing that the non-infallible Magisterium was non-infallible (because they defined authentic Magisterium as = the non-infallible Magisterium). So this was a 60-minute video arguing over nothing more than the definition of a word.