Did the Roman concrete makers do this on purpose, or was it a fortunate result of imperfect manufacturing methods? it’s remarkable either way.
No, they did not “engineer” it.
They used all available ingredients and over time figured out what worked and what did not work.
That said, there have always been some people who were better builder than others. Throughout history.
We are not all alike and never will be.
The idiots now bowing to the great computer in the sky, will fail in the end.
God is in charge, not a stupid machine.
I've always admired Roman engineering, but there is simply no way any craftsman of the time would know how his work would withstand the millennia. Nor could he have relied upon the examples of deeper antiquity, as there was no communication of information from his occupational predecessors. There is a type of survivor bias with which we moderns must contend when evaluating what remains of a (previous) variated subject of study. Structures which remain are remarkable for having survived, while those which did not last are somehow NOT remarkable and not held as evidentiary.