Counterpoint: the investigators used a crossover design, where each participant acts as their own control. Since within-subject variation <<< between-subject variation, this makes the effective sample size much, much higher than it would seem at first. In terms of efficiency, their design’s potentially equivalent to a vanilla parallel-arm trial with close to 100 subjects (or more) in each arm. So, you definitely can obtain solid evidence from N=15 (or less), if your study is carefully designed.
Sample size is only a very small part of the overall methodological picture—design matters too.