Quicksort is often described as an in situ (in-place) algorithm, despite the fact that it requires O(log n) stack space. So does in situ mean "requires less than O(n) additional space", or does stack space generally not count as space complexity (but why would that be the case?), or is Quicksort actually not an in situ algorithm?
is Quicksort actually not an in situ algorithm?
The standard implementation of it is not in situ. It's a horribly common misconception, but you as correctly note due to stack space consumption, that conception is wrong.
I say "standard implementation" of it because people have modified the algorithm to make it an O(1)-space algorithm. Here is one example: Quicksort without a stack.
Of course, consistent with the classic space-time tradeoff, such versions of quicksort are less performant than the standard implementation.