It does explain why Brewster felt romantic inclination towards Caucasian woman accountant. It runs in family.

Yes it was ridiculous, but like Arnie and Devito in Twins, it added to the comedy.
"Look at it this way; in a hundred years who's gonna care?"

He takes about a full minute of time in the prerecorded video to explain why this is so. It almost seemed ridiculous.
I'm guessing that the book was written with the the main character being white. But since Pryor was cast, they had some explaining to do. I could be wrong, that's just a guess.

Does it sound like I watched the movie goddamit?

Yeah, don't you just love it when someone asks a question about a movie when the answer is explicitly said/explained in the movie. In this case it was in the first 20 minutes of the movie.

Uncle Rupert explains it all.
Good God Pay attention.

His great uncle was the half brother of his grandmother, his uncle was pure white, his great grandfather then had his grandmother with a black woman and I assume she had children with a black man and so on.