Yes, John, correct. Maybe hip mobility (rather than flexibility) would be a better term, implying range of movement.
But also strong muscles of the pubic area (beneath the waistline) and obliques/laterals of the back/sacrum area. Essentially the lower core. Awareness and understanding of the role played by keeping hips in neutral would also be helpful so as when the athletes goes through her/his mental checklist in training the positioning can be consciously implemented. Hopefully it will eventually happen without thinking during the major races in due course.