Yeah, these kind of statements feel very “empty” to me. That’s probably true that you could do everything with Nim, it’s also true that you can do everything with every language. Should you? That is another question.
Unfortunately, it's not possible to do everything with every programming language. If thinking of rewrite of some software, it depends does all dependencies, database drivers, authentication, etc exist for some other programming language.
Being able to compile to C, C++, ObjC, and JavaScript natively (and LLVM using https://github.com/arnetheduck/nlvm ), along with an excellent FFI (including to and from Python) means you don't need to rewrite dependencies as you can use them directly. Nim is great at glue code - arguably better/easier than Python.
Along with general language characteristics such as being high level and productive like Python, but with intricate "bare metal" control when you want it, really does make it suitable for writing almost everything.