Sephiroth had no intention of disarming Seres by any of the means he had suggested to her just a moment ago and held his sword still while he watched her go through the movements of what he had just demonstrated.
When she was going through her own re-enactment of his performance, he dramatically dropped his sword when she came in for the disarm, smirking.
“My left hand is already missing.” He paused. “So, partial credit, I suppose.”