So, this guy sailed in one of those round-the-world races, and he's a convinced Flat Earther.

Here he talks about how they use the (invented) concept of "magnetic declination" to fake a globe earth.  He also says that it's why people are not allowed south of 60 degrees latitude, because the declination model fails past that point.

I had never heard of this concept before, but evidently you can't just follow your compass but always have to make this fake adjustment for "magnetic declination" ... which has the effect of hiding the fact that you're stretching out in either direction.  I've always wondered how a compass could point North from the Southern Hemisphere if you're wrapped around a ball earth.

I haven't watched the video yet, but have you heard of the magnetic poles moving over time? Maybe that could be part of it? I don't claim to know anything about global geology and the causes of the magnetic field, but I know magnetic fields are weird and complex in my opinion and experiences, so I wouldn't give any weight to a magnetic field proof of FE or GE either way.