When Atticus tells Scout that is she deletes the adjectives she will have the facts, he is telling her to ignore the embellishments in Jem’s story.
Jem likes sixth grade. He shares interesting things he learned about Egyptians with Scout, but he exaggerates a little and she has no idea what he is talking about.
He declared Egyptians walked that way; I said if they did I didn't see how they got anything done, but Jem said they accomplished more than the Americans ever did, they invented toilet paper and perpetual embalming, and asked where would we be today if they hadn't? (ch 7)
Lee includes this story because it does not just refer to Egypitans. Jem is growing up, and growing apart from Scout. He is starting to make less and less sense to her. Atticus is trying to warn Scout not to pay too much attention to him.