Why convert benzoate in soda to benzoic acid for spectroscopy experiment?
Your assumption is correct, the addition of HCl, as already in the earlier publication by McDevitt (J. Chem. Educ., 1998, 75, 625-629 DOI: 10.1021/ed075p625) ensures benzoates are converted into benzoic acids. 

The publication by Guo et al., is instructive as it lists several benzoic acids side by side, for example: 



source: J. Phys. Chem. A, 2012, 116, 11870–11879 DOI: 10.1021/jp3084293

The shift of the centre of absorption may be more important than a potential change in $\varepsilon$ if you want to use the UV-Vis spectrum recorded in a principal component analysis (PCA), where overlap of absorption bands of your consitiuents in the analyte should be avoided.