The title seems like a stretch since they then go on to say, "Audio samples were analysed objectively using acoustic analysis for measures of timing, vocal control, and quality. Subtle differences in speech timing, vocal effort, and voice quality may exist between cannabis and control groups, however data remain equivocal." "After controlling for lifetime alcohol and tobacco use and applying a false discovery rate, o…
Beyond a larger sample size, IMO it would have been also been useful to take repeat measurements of the cohort over time and also under the influence, especially given the observation of "change in line with duration of abstinence from cannabis use".
Perhaps those details are in the full paper, I haven't been able to find a copy.