You sort only less_than_head (BTW Your head is commonly called pivot) and greater_than_head is passed as is, so there is only partial sorting (only elements less than head are sorted).
EDIT: The big pro of using :lists.partition/2 is that it will split your list into two in one “walk” through the list instead of two, which can be improvement in case of long lists (but in such case the quick sort isn’t the best solution anyway, merge sort will work better).
So your 2 comprehensions can be easily replaced with:
{lesser, greater} = :lists.partition(& &1 <= head, tail)
