I believe that Biblical history takes man back about 6,000 years or so. Many evolutionists take man's origin back millions of years.
That both sides would agree that man had full intellectual powers six thousand years ago means absolutely nothing to the debate. (Though to people of faith there is no 'debate' about creation).
'Debate' about man's origin only occurs when those who attempt, with junk science, to disprove the Bible's teaching. Though the theory of 'evolution' is unobservable and utterly impossible to recreate, it is advanced like some sort of secular catechsis or unassailable doctrine.
Actually it makes your earlier argument moot:
Since I never subscribed to the theory of evolution, I believe man's immediately-created intelligence allowed a written language to begin very rapidly
Both you and those who accept evolution agree that man had full and complete intelligence 6,000 years ago. So any concept of "gradual intelligence" is totally unrelated to the orgins of writing.