USB charger limits: 9 V, 1.7 A but only 5 V, 2 A? Not exactly anti-proportional
Chargers with variable voltage output (PD or QuickCharge) use some switcher-based converters, which involves an inductor and active MOSFET switch. The converter is usually the same for 5V and for 9V, only the set point gets changed in accord with link negotiations.

If the inductor/switch is economically designed to handle 2 A (to provide 9V at 1.67A), it can't deliver more than 2 A when the set point is changed to 5 V output. That's likely why the manufacturer limited the 5V-output to 2 A only. It looks like this design is limited on DC side, and expectations to have "proportional" 15W-output in both modes are unfounded.