It is probably more of an inference than a direct quote sort of thing. Most of the races have a racial skill with some weapon or other. Since Hyborians are usually fighting armoured foes (knights, et. al.), it would make sense they would prefer a heavier, armour piercing weapon over a light rapier-style arming sword. It may well be taken from the Nemedian Adventurer in The Hour of the Dragon, who had a great-sword. It is probably there for game balance against the other races more than anything else.
Does it really matter if there is a direct quote or if it is just inference or game balance? If you don't like it, don't use the rule. I have yet to see an Aquilonian PC really take up the Great Sword if the Player envisioned the character fighting with a different weapon. It seems like a strange aspect of the rules to be focused on.