NO. they haven't mentioned that in the problem. but in editorial they return -1 for root == Null because:

def height(root):
    if root:
        return 1 + max(height(root.left), height(root.right))
    else:
        return -1

because they return 1 + max(root.left+root.right) now if root.left or root.right were null then max(blah blah) will return -1 and together they would return 0 as height of that root without left or right subtree..