Scalability versus Responsiveness in .NET Async

I'm reading through the Exam Ref 70-483: Programming in C# book and the following code sample is given:

LISTING 1-19

public Task SleepAsyncA(int millisecondsTimeout)
{
    return Task.Run(() => thread.Sleep(millisecondsTimeout);
}

public Task SleepAsyncB(int millisecondsTimeout)
{
    TaskCompletionSource<bool> tcs = null;
    var t = new Timer(delegate { tcs.TrySetResult(true); }, -1, -1);
    tcs = new TaskCompletionSource<bool>(t);
    t.Change(millisecondsTimeout, -1);
    return tcs.Task;
}

The paragraph under states this:

The SleepAsyncA method uses a thread from the thread pool while sleeping. the second method, however, which has a completely different implementation, does not occupy a thread while waiting for the timer to run. The second method gives you scalability.

Why is A responsive but B scalable?