This only flattens one level. Consider the following snippets:

    ; CHICKEN Scheme
    #;1> (flatten '((1 2 3) ((4 5) 6) (7 (8) (((((9))))))))
    (1 2 3 4 5 6 7 8 9)
    #;2> (apply append '((1 2 3) ((4 5) 6) (7 (8) (((((9))))))))
    (1 2 3 (4 5) 6 7 (8) (((((9))))))

vs.

    # Python 3
    >>> sum([[1,2,3], [[4,5],6], [7, [8], [[[[[9]]]]]]], [])
    [1, 2, 3, [4, 5], 6, 7, [8], [[[[[9]]]]]]

There's a big difference here. Flattening a list to just the elements inside isn't terribly hard, especially in a language like Scheme with tail-recursion, but flatten is definitely something that should be in the standard library. The "flatten" you propose is really just appending the elements of the first level of the list.