What effect has the movement of the African tectonic plate to the NE over 4,600 years had on these calculations?

The African Plate's speed is estimated at around 2.15 cm (0.85 in) per year. It has been moving over the past 100 million years or so in a general northeast direction. So around 90-100 meters.

Huang, Zhen Shao (1997). "Speed of the Continental Plates". The Physics Factbook. Retrieved 7 June 2018.