I’m rather doubtful about this. Granted, the Egyptians didn’t leave much of a record about how they built anything, but concrete is supremely useful. If they knew how to make it, and did use it to make blocks, why didn’t they use it for buildings, bridges, and temples? It also seems strange for something like that to be lost. Builders who knew how to use it would almost surely have passed the knowledge down, even between the stable Kingdoms.
Either the claims that Egyptians used concrete are wrong, or they really didn’t know what they had. Or everything else they built out of concrete was lost to history, which is improbable. Until there’s some more confirmation, I’m going to be doubtful about this claim.