The statement isn't very accurate. I live in a house that has a view of the sunsets from my porch. After just one or two years It was easy to find where the end towers would be placed by just watching the sunsets from my porch. Since the towers seem to be evenly spaced, it would then be just a matter of spacing them between the end points.
However, the sun doesn't move across the horizon at an even pace. The distance changes very slowly at each end point, and most quickly at the solstices in the middle. The towers would have to be spaced accordingly to make up for this. That would make the tower spacing very complicated without algebra and geometry.
“The distance changes very slowly at each end point, and most quickly at the solstices in the middle. The towers would have to be spaced accordingly to make up for this.”
They also observed the moon. Making note of where the sun sets on a full moon would accurately give you the next monthly tower location in between those end towers.
Something to consider is the time frame. These were probably the accrued observations and knowledge of several generations passed down. They didn’t just go out with a slide rule and build these in the correct places in just a few weeks. They had time to observe and adjust until more accurate before building them permanent.