FWIW, the "catenation operator" in the Nim stdlib is ampersand `&`, not `+` which actually makes it better than most PLangs at visually disambiguating things like string (or other dynamic array, `seq[T]` in Nim) concatenation from arithmetic. So, `a&b` means `b` concatenated onto the end of `a` while `a+b` is the more usual commutative operation (i.e. same as `b+a`). Commutativity is not enforced by the basic dispatch on `+`, though such might be add-able as a compiler plugin.
Mostly, it's just a very flexible compiler / system.. like a static Lisp with a standard surface syntax closer to Python with a lot of parentheses made optional (but I think much more flexible and fluid than Python). Nim is far from perfect, but it makes programming feel like so much less boilerplate ceremony than most alternatives and also responds very well to speed/memory optimization effort.
For example, in Nim, at the compiler CLI tool level, there is opt-in/opt-out via the `--mm=whatever` flag, but, at the syntax level, Nim has both `ref T` and `ptr T` on equal syntactic footing . But then in the stdlib, `ref` types (really things derived from `seq[T]`) are used much more (since it's so convenient). Meanwhile, runtimes are often deployment properties. If every Linux distro had their libc link against -lgc for Boehm, people might say "C is a GC'd language on Linux". Minimal CRTs vary across userspaces and OS kernel/userspace deployment.. "What you can rely on/assume", I suspect the thrust behind "optionality", just varies with context.
Similar binding vagueness between properties (good, bad, ugly) of a language's '"main" compiler' and a 'language itself' and 'its "std"lib' and "common" runtimes/usage happen all the time (e.g. "object-oriented", often diluted by the vagueness of "oriented"). That doesn't even bring in "use by common dependencies" which is an almost independent axis/dimension and starts to relate to coordination problems of "What should even be in a 'std'-lib or any lib, anyway?".
I suspect this rule is trying to make the adjective "GC'd" do more work in an absolute sense than it realistically can given the diversity of PLangs (sometimes not so visible considering only workaday corporate PLangs). It's not always easy to define things!