Can I prevent a 9V LED flickering when dimmed to around 30%?
There is no inductor on the dimmer PCB so it is almost certainly driving the LED directly with the PWM signal.

If you attempt to filter the output just with a capacitor it will probably cause the LED to run at almost full intensity even if adjusted for a lower value. The capacitor will charge during the on period and then power the LED during the off time. It will also cause very high peak currents in the dimmer that could potentially damage it.

To avoid flicker you would need to provide the LED with DC. Your big resistor suggestion would do just that.