There is no proof that there is a molten outer core, nor a solid inner core for that matter. The structure of the earth is a theory that is supported by observable facts. E.g. Seismic surveys, the behaviour of the magnetic field, volcanic activity, tectonic plate movement, and so on. Theories change as our understanding of the planet gets better and technology improves. 40 years ago we believed the earth had a molten inner core and a solid outer core.
Anyway, I digress. The question I posed was how does the flat earth theory explain how the magnetic field of the earth flips ?