I think you did misunderstand their point. Browser native doesn’t just mean DOM, there’s also Canvas and directly reading mouse offsets and various other approaches. What they mean is don’t transit a mouse click through an OS emulation layer through a hardware emulation layer to the simulator, just directly read input from the browser and skip all the intermediate layers and inject the value directly to the code. Imagine that the mouse is simulating a PS2 hardware device with input polling and interrupts on top of a whole simulated OS and all that crap.

The approach suggested is assuming you have access to the code to modify how it gets input data, or you’re willing to update the memory directly when input happens.