Is std::ref really necessary when passing references to threads?

I'm reading through C++ Concurrency in Action and in Chapter 2 I'm led to believe that even if a function prototype, e.g:

void MagicFunc(Data& myData);

is intended to be used like the following:

Data dataExample;
thread t(MagicFunc,dataExample);

I should really do this

Data dataExample
thread t(MagicFunc,std::ref(dataExample));

or otherwise the changes I expect to have happened to "dataExample" won't have take place. Specifically it states something like:

Although MagicFunc expects the second parameter to be passed by reference, the std::thread constructor t doesn’t know that; it’s oblivious to the types of the arguments expected by the function and blindly copies the supplied values. When it calls Magicfunc, it will end up passing a reference to the internal copy of data and not a reference to data itself. Consequently, when the thread finishes, these updates will be discarded as the internal copies of the supplied arguments are destroyed, and process_widget_data will be passed an unchanged Data myData rather than a correctly updated version.

However, testing this out with the following program

#include <iostream>
#include <thread>
#include <vector>
#include <chrono>
#include <assert.h>
using namespace std;
using namespace std::chrono;

const int NUM_VALS = 50000000;

#define _MULTICORE 

void AddValuesToSlots(vector<int>& vecVals,vector<int>::iterator& begin,
                      int num,int startNum){
    int i = startNum;
    auto end = begin + num;
    for (auto itr = begin; itr < end; ++itr){
        *itr = i++;
    }
}

int main()
{
    vector<int> vecVals;
    vecVals.resize(NUM_VALS);

    //get number of cores and divide up the workload
    unsigned int numCores = thread::hardware_concurrency();
    unsigned int slotsPerThread = NUM_VALS / numCores;

    //for timing
    high_resolution_clock::time_point t1 = high_resolution_clock::now();


    thread* t = new thread[numCores];

    //get the iterator to the beginning
    auto begin = vecVals.begin();

#ifdef _MULTICORE
    for (int core = 0; core < numCores; ++core){
        t[core] = thread(AddValuesToSlots, vecVals, begin + core*slotsPerThread,
            slotsPerThread, core*slotsPerThread);
    }

    for (int core = 0; core < numCores; ++core){
        t[core].join();
    }
#else
    AddValuesToSlots(vecVals, begin, NUM_VALS, 0);
#endif


    delete[] t;

    //how long did it take?
    high_resolution_clock::time_point t2 = high_resolution_clock::now();
    cout << duration_cast<milliseconds>(t2-t1).count() << endl;

#ifdef _DEBUG
    //test that the values are correct
    for (int slot = 0; slot < NUM_VALS; ++slot)
        assert(vecVals[slot] == slot);
#endif

    return 0;
}

I've tried encasing vecVals in a std::ref and without, both times it executes without problem. Is the std::ref then really necessary and the information provided erroneous?

Thanks