That wouldn't put him in her guard. She was in his guard, then essentially he sat up as she leaned back. Her legs would never have had a chance to get inside his to where she could pull guard, unless she used a maneuver you haven't described. He never fully stood, only sat up and moved forward to sit on her stomach with both her legs behind him, since he was already between them and didn't have to fight past them. Keeping her legs apart and in the air would just leave them spread out behind him with him still sitting on her stomach.