“Know, O Prince, that in between the years when the oceans drank Atlantis, and the rise of the sons of Aryas, there was an age undreamed-of, when shining kingdoms lay spread across the earth, like jewelled mantles beneath the stars.” Etc.
Was that part of Robert E. Howard’s original work, or did deCamp and Carter add it later? If it was a later addition, was it written by deCamp, or by Carter?
If you’re looking for an in-universe answer, the current Dark Horse comic suggests that Thoth-Amon is responsible for the translation, as he is reading the newly-discovered exploits of the already-lost-to-history Conan to his Prince (the Prince of the opening lines). But, otherwise, yeah, it’s a Howard original.
That’s not Thoth, it’s a character named just “The Wazir” IIRC. In universe Thoth was killed by Prince Conn in the King Conan era - long before the "Know ye, oh Prince . . . " of the Dark Horse Conan series.