Are C/C++ considered one skill in the job market?

The answer is a big No

first C/C++ are mostly used in an an internal part of a software project, Most MNC and MAANG companies rely on C for making their own CLI tools, which are tools of API to make it better.

So the ratio of the hiring is meagre and they won't consider only the person with skill or experience for C/C++, they will expect additional skills from you.

Example: the Embedded Software to manage Google Cloud platforms is built in C++, so they need a person with Embedded knowledge and cloud operations procedures.

Also recently most companies started moving, migrating towards the RUST the alternative for C and faster than C, most companies replacing their C with RUST.