Seath didn't create sorceries, he created crystal sorcery, which is basically an advanced version that makes already existing spells even more powerful. Sorceries were already a thing back in Demon souls (which many theorize is the world before the age of dragons) but no one really remembers that. As for the Moonlight Greatsword itself, the sword existed waaaaaaay before the dark souls or even the demon souls era. It's actually a weapon that originated back in King's field, the first game made by FromSoftware. The version of the sword that you see being used in the spell is actually the model used in King's field and in Demon Souls, A.K.A the MLGS's "original" form before DS came to be.