Quote Originally Posted by thubby View Post
because sephiroth isn't the one the title is referring to.
Actually, it is.
Sephiroth's "One-Winged Angel" incarnation and its six white wings are a direct nod to the angelic description of Seraphim in Isaiah 6:2.

"Above it stood seraphim; each one had six wings: with two they veiled their faces, with two they veiled their feet, and with two they hovered aloft".

However, the name and title "One-Winged Angel" owe themselves not to these six white wings, but to the seventh single black wing.