Experimental bias?
"We recruited participants for our study through Prolific between December 2023 and February 2024, under the criteria that they were 18+ years old and located in the US. [...] The reference category is a Human-Human debate carried by a Male, aged 18-24, White, with a High School education, Employed for wages, Democrat."
While not immediately obvious what the overall spread around the reference category was due to the number of parameters, some of which are interdependent (see fig D4 in paper), the population of subjects is clearly not universally representative, so the study is only really valid for the population in question.
Apart from which, in an age of increasing verbal incoherence ("sort of like y'no") even on the part of supposed experts presenting on the media, the common level of persuasiveness is so low it would be hard for it to exceed that of a machine generating the most statistically likely word stream.