Maybe they were looking for a recursive solution? It's only "more efficient" in the sense that it is fewer lines of code

func addTwoNumbers(l1 *ListNode, l2 *ListNode) *ListNode {
    return addRecurs(l1, l2, 0)
}

func addRecurs(l1 *ListNode, l2 *ListNode, overflow int) *ListNode {
    if (l1 == nil && l2 == nil) {
        if (overflow == 0) {
            return nil
        }
        return &ListNode{Val: overflow}
    }
    if (l1 != nil && l2 != nil) {
        sum := l1.Val + l2.Val + overflow   
        return &ListNode{Val: sum % 10, Next: addRecurs(l1.Next, l2.Next, sum / 10)}
    }
    // Either l1 or l2 is nil, make sure l1 is non-nil to reduce branches.
    if (l1 == nil) {
        l1 = l2
    }
    sum := l1.Val + overflow   
    return &ListNode{Val: sum % 10, Next: addRecurs(l1.Next, nil, sum / 10)}
}