There is no real saying how they got them, the Ahamkars can control people,, it is possible the items are not used by them but were gathered by old/ slayed guardians or maybe that is what Osiris or the Jovians are off doing, collecting the items for the ahamkaras to sell to the gaurdians through Xur. Just like you Theory this one cleary hs its own holes, such as why and how to the Ahamkara's have these gun's. also Ahamkara's are believed to be extinct however I i recall there is a card that goes into stating how they are known to defy death and/or made the tower/guardians believe they were extinct so they could escape.