I feel like they were a bit ambitious with their presumptions.
And with the way the Egyptian gov is, we will likely never know.
The most likely explanation for what the pyramids were, based on what I have heard speculated over the years, is power and/or chemical plants. No mummies have ever been found in these, they were not tombs.
Resonant cavity, water below the pyramids would create a piezoelectric effect, the residue on the walls had traces of various elements due to chemical reactions, and the composition of the stone can all be used to explain quite thoroughly how these could have been used for power generation.
All the ancient monolithic structures are so incredibly impressive.