That claim isn't made on the linked site, though. I think people who write C nowadays tend to be working in the drivers and OS development spaces.
- People who work on projects which require low-level access, potentially mixing in assembly code (drivers, OS, etc), porting to different architectures, real-time, high-precision, control of timing, lack of MMU, etc
- People who want to perform specific actions, like a specific OS syscall, that may not be exposed [easily] by a higher level language in the way the programmer wants
- People who want to write very fast/efficient code
- Legacy code maintainers
Not too long ago I busted out some C to create a stripped binary for a job interview round. The interviewee needed to diagnose what the binary was doing and why and report back. Not all that common for most programmers today, but I expect a good amount of detail from a senior-level systems candidate.
I think it's also students; I think some people are still taught C as their first programming language. If you go on r/C_programming on Reddit, a lot of it is filled by very basic questions from what seem like beginners; I suspect they're students trying to do their homework.