Science fiction usually carries with it a desire to rationalize and explain the technology it's built upon, to try and paint a world plausible from a scientific standpoint. You see this a lot with the technobabble in Star Trek.
Cyberpunk has a lot of overlap with science fiction, but usually dives more into the social commentary on society and capitalism, using the technology within as a vehicle to amplify those criticisms. Some cyberpunk works seek to explain their technology and make it seem grounded in the same way sci-fi does, but that is usually secondary to the social and political themes.
You're being prescriptive and not descriptive with the definitions. Superficially it is the case, and people have created a neat little categorical hierarchy you can keep pointing back to, but I'm telling you that a lot of cyberpunk creative work is sci-fi in the same way that people say Star Wars is sci-fi (it's a space opera, at least the movies are)