?
Nim is compiled into C, IIRC, but it uses Nim syntax and rules and you don't call it "C". Same way, Python's Numba is compiled into LLVM IR at runtime, and you basically write Python, with a few limitations. You don't have to write or know any C/C++ to write high-performant numeric algos in numba (just clarifying) and there is no "C implementation underneath" as you're saying. It's also one of the easiest ways for "AI researchers in universities to do the work" because they and their friends probably already know Python but not Nim.
Re: ML libraries like lightgbm and many others - they are written in C++ so that there's a public C FFI which can then be wrapped in other languages, and not only in Python. This is probably the most flexible way to do things as opposed to limiting the whole thing to one niche/language.
// I'm not saying Nim is bad, Python is good, or any of that - I like alternative languages myself, but being an "AI researcher" and practitioner and spending most of my work time on developing numeric ML algorithms, I'd never look into Nim for doing any serious work, at least not now, partially because then I'd be the only person maintaining whatever I write alone and forever.