He's referring to spatial complexity of sorting algorithms. For example, mergesort can be implemented in parallel and in place (O(1) spatial) easily using link

He's referring to spatial complexity of sorting algorithms. For example, mergesort can be implemented in parallel and in place (O(1) spatial) easily using linked lists. Introsort and quicksort can be done in place using an array, but constant spatial complexity isn't always priority one. Some of the more recent algorithms aren't constant (timsort, spreadsort). Linked lists (doubly linked lists) still have valid uses, but it's generally better to use an array (or vector) until you can justify otherwise.