Sometimes teams are looking for specific knowledge or experience, then it really does matter. Other times, they are looking for a good developer who can pick up the necessary knowledge, and then it's much more open.

For example, I don't care what programming language you use in your interview. Senior engineer using Python? Sure! Want to implement in low-level C? Sure! I know any good engineer can easily be productive in a new language quickly.

But if I'm hiring someone to build/extend a real-time OS, then you need to check the "has experience working on real-time OS" box, because that's what I'm paying for when I hire you.