The 'x' is the usual symbol to indicate breaking a circuit, e.g. by removing a wire or cutting a PCB track. You could power the flasher unit downstream of your switch as you suggest but you should make sure to break the existing switch-to-light circuit. Otherwise, your landing light switch becomes a 'bypass' switch and, not knowing the details of the flasher unit, it's inclear whether it would appreciate that configuration.
One advantage of doing what you suggest might be that you can leave the fiddly rotary switch in your favourite position and control the landing light with a single flick of the rocker switch. But I daresay there are placarding requirements and maybe STC (or whatever) compliance factors to consider in a certified aircraft. Even if you did the factory recommended installation and left the landing light switch physically in place, I guess you need to placard it as u/s.
It might all be bit disco in practice! Any evidence that flashing the lights scares livestock?