I've had my inermis for 8 years, and it has been growing segments regardless of whether or not it gets watering in fall and winter. However, my paper-spine Tephro is reliable about growing 1 or 2 new segments per year, and the inermis is definitely slower in that regard. I believe keeping it plump with deep watering in winter is the main reason why it doesn't drop segments, but from what you just said, your inermis probably doesn't need any water at all during wintertime.
By the way -- comparing notes with the experiences from other growers, paper-spine Tephros are more fragile than inermis (AKA the "pine cone" cactus), and the best way to keep paper-spines intact is to have them in a greenhouse so they're not disturbed by the wind. Unfortunately, I can't give mine that protection, so all it takes is a good stiff breeze (plus the forces of gravity), and the segments drop, then into the trash they go.