>>On the winter solstice of December 21, the shortest day of the year in the northern hemisphere,[clarification needed] the shadow of one of the blocks disappears when the sun is directly above it.

This is not quite right. On that day the Sun is directly above São Paulo.

At the archaeological site: Latitude = +2.62°, Solar declination ≈ −23.44°, Difference = about 26.06°.

Therefore, at solar noon: Sun altitude ≈ 90° − 26.06° = 63.94°. Consequently, at that particular latitude, a normal vertical stone cannot have its shadow disappear because the Sun is directly overhead on December 21.