I don't think their "entire C++ language" wording also includes the standard library (except maybe for algos with std::execution semantics). I've never written Vulkan code but I do a lot of CUDA (and now HIP) GPGPU stuff and things like template meta-programming are immensely useful to have generic kernels with stuff like compile-time strategy pattern to avoid lots of mostly duplicate kernels, unrolling and classical loop patterns like tiling etc... can be easily refactored into nice looking metafunctions with zero runtime overhead. It's a game changer when you have access to modern C++ features to do GPU programming.