The Sun does orbit the galaxy. And it orbits the Earth too from this frame of reference. Another way of looking at is that the Sun and Earth co-orbit the galaxy while orbiting each other. Technically the Earth doesn't orbit the Sun either. The
barycenter of the solar system is a point
close to the Sun. Both the Sun and Earth (and all other planets etc.) orbit this point.
All this stuff about what orbits what is just splitting hairs. The Koran accurately describes what the Sun does relative to the Earth, which is what mattered to the people of the time.
But more importantly the Koran provided moral guidance and a legal framework that helped maintain a stable society. You could say why have a God then, and the answer is that a higher authority was needed - one that couldn't be corrupted by every power-seeker trying to twist the law to suit himself. Having a monotheistic god was a big improvement too, over the multiple gods and other magical stuff people were constantly inventing.
Sure it would be better for rationality if the Koran went totally secular, but that wasn't going to happen back then - and the World still isn't ready for it now. It's not that long ago that Christianity was in the same place. In the 1960's when I grew up practically everyone believed it (or least pretended to) and it didn't cause any great harm because most people understood its purpose.