> Why, exactly, is the c++ std::sort "wrong"?

It's silently an unstable sort, which is surprising, and then to add insult to injury it's also slower. Yeah, I know, the C++ unstable sort is so slow it's slower than Rust's stable sort.

YMMV for input types, sizes etc but generally that's what the numbers look like and though it's not universal it's actually quite common. "I bet the C++ is faster" is the wrong instinct, sometimes by a large margin.