You have a point I will have to think through.
It’s true that the Romans used concrete extensively in their construction in buildings such as the Pantheon. Without cement, many of the Romans engineering feats would have been impossible.
The ‘secret’ was lost until the 1700s when an Englishman ‘invented’ the same process. A later refinement, Portland Cement, was invented in 1824.
However, I am unable to explain how the ancients were able to sculpt and cut geometric figures and stela with straight lines back in a time when the only tools available were harden sticks and stone hammers.
There is a S. American site (on top of another mountain) that has what I would call ‘shadow boxes’ for want of a better word that have multiple levels of straight lines incised into granite walls and standing columns.
When thinking of ‘lost’ knowledge, you have to wonder about how they acquired the knowledge and tools to accomplish the tasks—and why none of the masonry tools have been found.
I suppose another possibility would be if the original inhabitants were driven away by warfare and the city were taken over by less skilled people.