I’ve never heard that before either.
However, I teach the STOTT Pilates approach to pilates and the rollup is a movement that is taught progressively.
It can be taught as a:
1. Half roll up with the knees in flexion.
2. Half roll up with the legs extended.
3. Full roll up with the legs flexed.
4. It can be performed with a dowel
5. It can be performed with a ball between the knees.
The point is to modify the movement according to the level of your class or individual client.