Why the FFT result frequency magnitude does not be mutiplied by 2?

Hi everyone,
As for the FFT function:

void FFT::performFrequencyOnlyForwardTransform (float* inputOutputData) const noexcept
 {
            if (size == 1)
                return;

            performRealOnlyForwardTransform (inputOutputData);
            auto* out = reinterpret_cast<Complex<float>*> (inputOutputData);

            for (auto i = 0; i < size; ++i)
                inputOutputData[i] = std::abs (out[i]);// After the abs, it does not bultiplied by 2

            zeromem (&inputOutputData[size], sizeof (float) * static_cast<size_t> (size));
}

From the index N/2+1 to N-1, the FFT result F[index] is the Conjugate complex number of the index 1 to N/2-1. F[index] (index N/2+1 to N-1) is just the negative frequency. After get the absolute value,why it does not be multiplied by 2?

In the Tutorial: Visualise the frequencies of a signal in real time, after get the FFT result, the absolute value is still not be multiplied by 2.
why?
Best regards!捕获

In the MATLAB fft help page example, it be multiplied by 2.