You are referring to the MA (Cantab.) / MA (Oxon.) which are conferred to BA graduates six to seven years from matriculation, and indicates rank within the university. This is why I said Cambridge _didn't_ offer a Master's in Computer Science.
The MPhil/MEng are absolutely real qualifications, requiring an additional year's study and a substantial research project.
I studied at Cambridge, and I have _never_ heard "Master's in X" used to describe the MA (Cantab.). It's simply wrong - the MA is not a subject specific qualifications - and can be trivially checked by employers - neither degree certificate (the BA or MA) states subject[1], and the degree transcript will indict you only took undergraduate level courses. As a result, the phasing is typically "I have an undergraduate degree in X"/"I did my undergrad. at Cambridge".
If people are using it to be misleading, that isn't really Cambridge's problem -- anywhere it matters, people can ask for proof (the transcript), and there is a fair chance the person is being misleading in other ways as well.
[1] Example BA certificate: https://i.stack.imgur.com/VTWUb.png All undergraduate degrees are the Bachelor of Arts, regardless of subject (except Law, which is the LL.B.).
The MPhil/MEng are absolutely real qualifications, requiring an additional year's study and a substantial research project.