The original intent of the writer for the Titan Mauler archetype was to have characters wielding Huge, Garg, and Col weapons. But the balance team decided that was excessive and deliberately changed the wording so that it doesn't exempt the character from the size-step change inherent to wielding wrong-sized weapons. It wasn't a "blunder", it was a deliberate change to address a perceived balance issue.
Also, don't trust Thax; he has a history of presenting disinformation specifically on the subject of wrong-sized weapons. The correct interpretation of the rules is as follows:
All weapons have an "effort-to-use" category and a "size" category. The effort to use is whether it is a light, 1-h, or 2-h weapon and the size is the size of creature it was designed for. So a Medium Two-Handed weapon is a weapon that requires Two-Handed effort by a Medium creature (or one that counts as Medium for wielding, ie. Centaur) while a Large One-Handed weapon is a weapon that requires One-Handed effort by a Large creature. If you wield a weapon for a different-sized creature, you shift the effective "effort-to-use" category by one step per size category difference. For instance, a Huge Light weapon wielded by a Medium character is treated as Light>1-H>2-H, a Two-Handed weapon and also suffers -2 attack per category change (net -4 for this example). It works the other way, as well; a Medium Greatsword wielded by a Huge character is treated as 2-H>1-H>Light>, a Light weapon, also with a net -4 to attack. If this change would take you "above" 2-H or "below" Light, you cannot wield the weapon. If a feat or ability changes the "effort-to-use" category and doesn't specifically call out that it applies only to weapons properly sized for you, then that means it affects weapons of all sizes and is then subject to category changes based on size disparity. In other words, a Huge Dagger is still a Light weapon, but a Medium character wields it "as if" it were 2-H. Thunder&Fang and Quarterstaff Master both specify you wield the Earthbreaker/Quarterstaff respectively "as one-handed weapons" meaning that a Large Earthbreaker/Quarterstaff is no longer categorized as 2-H>Unwieldable for a Medium character but, instead, is a 1-H>2-H weapon. Thax has a history of arguing against this principal of the rules and has been shown, time and time again, that he is incorrect but he insists on derailing threads on the subject; disregard anything he has to say on the matter.