I don't think the methodology is the issue with this one. 500 people can absolutely be a legitimate sample size. Under basic assumptions about the sample being representative and the effect size being sufficiently large you do not need more than a couple hundred participants to make statistically significant observations. 54% being close to 50% doesn't mean the result is inconclusive. With an ideal sample it means people couldn't reliably differentiate the human from the bot, which is presumably what the researchers believed is of interest.