(page 142, section 9)
Never mind that limestone in general has a fairly high resistance to begin with. It's stone for crying out load.
Regardless of the electrical resistivity of limestone, magnesium is a metal, and thus even trace amounts within stone will result in heightened conductivity. This is remedial physics. The Tura limestone of the casing bears however virtually no magnesium--it is closer to pure calcium carbonate. In any case, the fact that it is not all highly-conductive material merely serves to help store the electrical charge over time, until it eventually makes its way into the peak. By the time a decent electrical current had reached the gold capstone, it could have carried many millions of volts, based on the rate of conductivity, resistance, and the volume of the pyramid.