Not only would the natural ridges and outcroppings have had to be just as accurately aligned, but for either the natural or manmade structures, as the article noted, the onlooker has to be at a "specific observation point" for the time of year prediction to be accurate.
It seems implausible that along with some natural, but accurately shaped and positioned, ridges and outcroppings there was coincidentally also a specific natural sunset observation point from which one could obtain an accurate prediction of the time of year (solstices, equinoxes) to within two or three days
And still the question remains - how was this ancient civilization able to know that these natural ridges/outcroppings and observation point were accurate time predictors of an equinox or solstice worthy of duplicating with manmade structures?
It is not at all as complicated as you are describing. One can stand in front of the same large rock and observe a mountain range at different times of the year and make note of where the sun rises or sets along that range. Doesn’t matter how that range is as long as you observe it from the exact same place in front of that static rock.
One year of observation will set the static pattern and visual marker points along that range for Summer solstice and winter solstice as long as the sun is visible. Count days, divide, and make notes of all the points in between those two points on the range. You now have an accurate natural timepiece observable from that particular rock.
I did that with our local mountain ranges here. And from the same static observation point it is right on the money all year long. I moved or built nothing. Just observed and made mental notes from that one spot.
Here is one for you. Using our mountain ranges here I can also aim new satellite dishes precisely at the satellites on the horizontal alignment. Only have to adjust the vertical to be right on it. Accrued observation notes is all it takes.