Did you read the article?
The theory is that limestone concrete (limestone slurry mixed with lime from ash and salt) was used near the top of the pyramids. The pyramids were covered with a veneer of limestone slabs originally, IIRC, and are long gone. Recycled into other stuff.
I read it, and see it as bunk. limestone concrete is about as hard as classroom chalk, which is an extremely lousy building material.