I think this is an effect of perspective. Like if you look at street lamps at a far enough distance, the furthest ones will appear to be at eye level. They're not - it's just that's how they appear because of perspective. Say, measure a street lamp at 1m horizontal distance, it will be close to 90 degrees. Then at 10m - less. By the time you get to 100m, it will likely appear to be close to eye level. It's not, and it's nothing to do with curvature given the short distance - it's simply perspective.
Eratosthenes reasoned that the difference in the angle of incoming sunlight was due to the curvature of Earth's surface, and so by measuring this angle, he related the distance between Alexandria and Syene to the total dimension of the globe.
On the day the Sun shone on the bottom of the wells in Syene, Eratosthenes measured the Sun's position in the sky over Alexandria. It was seven degrees away from the zenith, meaning Syene must be seven degrees away from Alexandria as measured on the circle that is Earth's circumference. Because seven degrees is about one 50th of a full circle (360 degrees), Eratosthenes simply multiplied the distance from Alexandria to Syene -- believed to have been about 515 miles (830 km) -- by 50. He calculated Earth's circumference at 26,000 miles (42,000 km), only five percent away from the modern accepted value of 24,901 miles (40,074 km).
https://stardate.org/astro-guide/faqs/how-was-size-earth-first-measured#:~:text=On the day the Sun,circle that is Earth's circumference.
I would also add that for thousands of years man has been using a sextant to measure the angle of the Sun so they could know their latitude and find themselves on a map of the Earth.