Modern scholars are skeptical about what survives from Manetho's writings, nevertheless, it is quite apparent that the original Manetho frequently relied on legitimate and accurate source material about Egypt’s chronological history. In fact, he was a priest at the temple at Heliopolis and tradition held that in this temple there was a depiction of a Tree of Life that contained the names of all the Egyptian kings on its leaves. It has been argued,  in other papers and articles, that Manetho originally had a very accurate chronological history of Egypt and that the copies of his works that have survived represent badly garbled mutilations of his original text.

All this to say that modern scholars are willing to give the benefit of the doubt to Manetho.  Why not to the Bible which is infinitely more accurate?