Collections/Partition promotes inefficient code

[rwst]

The task description suggests to use partitions but it is wasteful because this should work too:

// Return customers who have more undelivered orders than delivered
fun Shop.getCustomersWithMoreUndeliveredOrders(): Set<Customer> =
    customers.filter {
        it.orders.count { it.isDelivered } < it.orders.size / 2
    }.toSet()

But it is not accepted:

java.lang.AssertionError: The function 'getCustomersWithMoreUndeliveredOrders' is implemented incorrectly