I agree with your assessment. ie, Induced magnetism.
That’s what they are talking about - induced magnetism. But it only gets induced when the rock is forming, or if it was heated and then cooled again. Hence fire-ring rocks can be dated to when they were last used in a fire.
So these guys are talking about when the concrete was formed. In geology you can see the magnetic change in the seafloor rocks from when they cooled. Also can see iron-rich sediments when they were laid down - as they settled into a lake they will orient to magnetic north.
However, based on only three locations I’m not sure that is conclusive. And I’m confused, it says:
“The block from one sampling location of the Khafre pyramid is of natural limestone and evidently comes from the adjacent quarry. It is likely that the block from one sampling position of the Khufu pyramid comes also from the same quarry.”
This makes it sound to me like TWO blocks were natural limestone - yet still had the same magnetic orientation. So either luck, or perhaps the builders were lining up the limestone to fit the original cuts. Although they said they were from three different locations. One would think they wouldn’t sample two blocks right next to each other, so that shoots out the fitting the original cuts. Although to me - that would make sense to do it that way. Cut a bunch of big ones for the base, line them up. Then some smaller ones for the next level, line them up. Etc.