As far as I can tell now, the pyramid shape is so that sideways
expansion of the soil/ice mixture above the frost line can't get a
frictional grip on the sides and pull it upwards as the soil/ice also
expands upwards. It's like using your 2 hands to squeeze a telephone
pole from the sides to get a grip while trying to lift it out. If the
telephone pole were shaped like a pyramid instead you couldn't get a
grip because your hands would slip upwards on it. This is also why
smooth concrete like that formed with Sonotubes resists uplift better;
it's too smooth to get a grip on.

Burying the bottom below the frost line means no ice can form to push
straight up on the base.

The very wide base also provides structural resistance to tipping,
since for the pyramid to tip over, the base has to be pushed down on
one side. The bigger the base, the more difficult it is to push it
down, since this requires compacting the soil under that side.

I'm thinking that the guy wire would have to be tied to the top of a
stake driven into the ground at an angle, so the guy wire would be
pulling directly sideways on the top of the stake. I've heard they do
this with circus tents.