"Yeah, I'm Sora. And you're the Witch of Time or something like that?" Sora asked.

Sephiroth wasted no time, and with his superhuman speed, went to try to immediately end the fight with a zantetsuken slice through Erika. All the while the stake stayed lodged in him, yet did not inhibit his fighting ability. 

Though he knew he couldn't remove it, it was as if he simply ignored it's effects. Was Sephiroth right in saying he was a being of pure will at this stage? could he truly ignore a manifestation of truth? depending on your view, this was probably due to how differently things were in his reality as compared to this one.