Why was it said in the interpretation of His Almighty's saying, "The sun should not overtake the moon," that each has its own light, even though science has proven that they are one?

The sun and the moon are independent entities, each moving in its own orbit, as Allah Almighty said: ﴿And it is He who created the night and the day and the sun and the moon; all in an orbit are swimming.﴾ The moon derives its light from the sun, and the sun is the source of illumination, as Allah Almighty said: ﴿It is He who made the sun a shining light and the moon a [reflected] light,﴾ where He distinguished between the radiance emanating from them. So, the sun is inherently luminous, while the moon is luminous by virtue of the sun's light. A lunar eclipse occurs when the Earth comes between the moon and the sun, because the moon's light is derived from the sun.