[Plimsoll]

The article reads, these pyramids were close to surface and therefore were found so early on.

As for how deep does heat actually go, I would say, that if earth wouldn't have any temperature of its own, it would go all the way down. The process just takes extreme amounts of time. Empirically it takes time for earth to freeze at winter (if you live north enough) and likewise one cannot dig the ground has melted at the late of spring. Other empirical example would be rock near a campfire, they will stay warm long time after the fire goes down. Earth is just a very big rock.

While the ground doesn't freeze at Egypt, they certainly have some seasons, with different average temperatures, to warm up or cool down the deep ground temperature. So within time they should be able to see temperature differences of objects buried deep into ground. Assuming the resolution of satellites is good enough.

Other question would be how much interference does the warm sand over these objects cause. As the sand is somewhat flat, it probably has black body like radiation curve and removing it should be easy, but there always will be some static from these processes reducing the total resolution.

Well all in all, I really don't know how they do all this.