HED recommends a 23mm tire for the Jet because tires of that size will combine with the external profile of the rim to form a nice aerodynamic shape.

The ETRTO chart is very conservative, and also fairly old. My guess would be that the minimum-width recommendations are based on assumptions which simply don't apply in current wide-rim road use cases, such as that the 21mm internal-width rim might not be designed to handle road inflation pressures, and that a user will pump their 23mm tire to 130PSI.