The background about the algorithm helps you understand why Bill is reluctant to go back and implement a new different drawing primitive. His reluctance is the reason why Steve needs to work hard at convincing him, which demonstrates the theme of the story.

I agree that it's probably too detailed in the first bit, but I don't see it as a major flaw for a series of articles about the development of the original Macintosh. The intended audience consists of hackers and developers who get intrigued by technical details.