A range of chemical reactions occurring on the surfaces of metal nanoparticles exhibit enhanced rates under plasmonic excitation. Recent analyses based on Arrhenius law fitting have argued in favor of a purely photothermal mechanism of enhancement and suggested the lack of an involvement of hot electrons. However, there is a caveat as shown here: under certain scenarios, it is practically impossible to distinguish between a photochemical (non-thermal) effect of plasmonic excitation and a purely photothermal one using a phenomenological Arrhenius fitting of the reaction rates alone.