"Constantine split the empire" is just incorrect. There were co-emperors with regional responsibilities both before and after him. They were co-emperors of one empire. If you want a firm split, 395 (death of Theodosius) is the more common date AFAIK.

I'm not denying that gradually, over well more than a thousand years, the empire (only surviving in the East) changed character in many ways. I am denying that a unique Eastern character sprang up (and immediately applied to Egypt) the moment that Constantinople was founded.