Yes the "Hyborian Age" of Robert E. Howard predates known history. I just always imagined Conan with a rough American manner of speech, so as to stand out from the civilized types, sort of in the manner of Kirk Douglas' Spartacus, where the gladiators were American (for the most part) and the Roman elites British (for the most part).
Unfortunately, this is something that many critics and lay (or non-Howard-fan) audiences would likely not be aware of. I'm somehow not surprised that Ebert was aware of it, though. He was a pretty informed fellow.
One has to believe that John Milius and company did more than a respectable amount of research into both the fictional continuity and "facts" and the real history of ancient barbarianism etc., and must have felt that Arnold's accent happened to match or just work very well with the character and landscape.