ColColton
[10] Knight
Not really.. Canonically, Soul Blade was Sophitia Vs Cervantes, SCI was Xianghua Vs Nightmare, SCII Soul Calibur was evil, SCIII and SCIV was Siegfried Vs Nightmare and SCV was Pat Vs Pyrrha, Nightmare working behind the scenes, being defeated halfway through and Soul Calibur (Elysium) being corrupt.The 'Light vs Dark' theme this series has is usually between Siegfried and Nightmare.
So no, the light Vs dark theme isn't usually Nightmare Vs Siegfried.. It's just the most recognisable one within the community.
However, I can see why people might think of Okubo's comment as a hint towards Siegfried, as like I said, within the community people generally recognise Siegfried and Nightmare as the main Light Vs Dark characters.
(By the way, hope I didn't come off as too harsh ^^)