How does "using std::swap" enable Argument-Dependent Lookup (ADL)?

In What is the copy-and-swap idiom this example is shown:

friend void swap(dumb_array& first, dumb_array& second) // nothrow
{
    // enable ADL (not necessary in our case, but good practice)
    using std::swap; 

    // by swapping the members of two classes,
    // the two classes are effectively swapped
    swap(first.mSize, second.mSize); 
    swap(first.mArray, second.mArray);
}

How exactly does using std::swap enable ADL? ADL only requires an unqualified name. The only benefits I see for using std::swap is that since std::swap is a function template you can use a template argument list in the call (swap<int, int>(..)).

If that is not the case then what is using std::swap for?

The "enable ADL" comment applies to the transformation of

std::swap(first.mSize, second.mSize);
std::swap(first.mArray, second.mArray);

to

using std::swap;
swap(first.mSize, second.mSize);
swap(first.mArray, second.mArray);

You're right, ADL only requires an unqualified name, but this is how the code is re-worked to use an unqualified name.

Just plain

swap(first.mSize, second.mSize);
swap(first.mArray, second.mArray);

wouldn't work, because for many types, ADL won't find std::swap, and no other usable swap implementation is in scope.