Time Auto-correlation function for a polar reutrn-to-zero waveform
The return-to-zero code spends half its time at zero volts, so half of the signal samples contribute nothing to the total of the autocorrelation integration. Therefore, if the pulse amplitude is &plusmn;A, then the autocorrelation peak will be \$\frac{A^2}{2}\$.

Page 10 in that paper is talking about the power spectral density, which is a different measure from the autocorrelation function.