I was going to go into detail about how absurd this intervention is in terms of scale of number of impacted neurons but really this sentence tells you everything you need to know about the quality and rigor of this work.
Why do you consider the composition of the participant group important and therefore insufficient? Stimulating the brain by ultrasound sounds like such a low level intervention that it is hard to believe that age or sex could matter a lot.
At my school volunteers were recruited for course credit. So it could be due to the elation of getting it over with.
I could also argue that the effect is due to the excitement of participating in a study. If the declared interest of the students is Psychology they might simply be excited to do the study.
There may also be a novelty component where the change in routine produces arousal.
Even n=1 can give a statistically relevant result (for the hypothesis that a given event never happens)
I haven’t read the paper, but n=51 can be more than high enough to get significant results for more realistic null hypotheses, if the effect is large enough.
Having said that, effects rarely are large. If you have a weaker effect, you need more subjects (picking a somewhat random paper from a google search: https://www.journalofcognition.org/articles/10.5334/joc.72/)
I’m surprised by the fraction of male subjects they managed to recruit. A well-known joke is that the typical experiment in psychology is either done with mice or with teenage girls as participants.