Took me a while to figure out whether it's interpreters for C programs or if there's a particular class of interpreters called "C". Turns out it's about interpreters implemented in C that they use modified LLVM to do the retrofitting, but couldn't it be applicable for other languages with LLVM IR, or other switch-in-a-loop patterns in C?
There were a couple of C interpreters since the 1990's, including with REPL support, but apparently never took off, most likely a community culture issue, that doesn't seem much value using them, beyond being in a debug session.
I've been a low level C and C++ programmer for 30 years. Even with your explanation and having read the webpage twice I have no idea what this technology does or how it works. So it takes normal interpreted code and jits it somehow? But you have to modify the source code of your program in some way?
You're quite right that since we're working with LLVM IR, adapting to other languages is probably not _that_ difficult, though these things always end up taking more time than I expect! Since the majority of real-world problems in this area depend on C interpreters, we put our limited resources to that problem. You're also right that "interpreters" is a pretty vague category, and there are other parts of C (and other) programs that could be yk-ified, though I suspect it would be a fairly specialised subset of programs.