Firesprite and Turbulent and have hired people specifically with Unreal experience, not CIG.
Those 2 companies have other projects beside SC. No wonder they want to hire people with knowledge of the standard engine in the industry.
Yeah, it makes sense because it may be a good proxy. Like, when you hire software engineers, you target experience in a range of applications of languages that are similar in philosophy or use cases to the ones you are hiring for, not necessary for the ones you are using. An experienced SE will pick up a new language, together in its tooling, in less than a month, because most concepts will be familiar. In fact, after the first week, this person should at least be able to read most of the code base, find anti-patterns, look for potential improvements etc. Unless it is something exotic, but even then it will be more about "a modern functional language" or "experience writing real time data pipelines".