If you actually look at the evidence presented, they cite this 2019 meta-analysis in the Journal of Management: “Making CEO Narcissism Research Great [ew]: A Review and Meta-Analysis of CEO Narcissism”. On signature size, for which it found only two studies by nearly the exact same team, it remarked:

Ham et al. (2018) and Ham, Lang, Seybert, and Wang (2017) employed an alternative unobtrusive measure of CEO narcissism by measuring the size and contents of a CEO’s sig- nature in SEC filings (2 of 42 articles). Their rationale posits that a larger signature represents the grandiose nature of a narcissist. To validate the measure, Ham et al. used student samples in a laboratory setting. While the main advantage of the measure is that it captures a behavior under the direct control of the CEO (i.e., his or her signature), the measure may not fully capture narcissism’s multifaceted nature. Ham et al. also provided external validation by cor- relating the measure with employee ratings of CEO narcissism, as obtained by O’Reilly et al. (2014). As this is a newer measure, it has seen limited use to date.

I still see this as some absolute TikTok narcissist-whisperer shit and emblematic of the worst of the reproducibility crisis and conclusion-chasing in the social sciences – sincere respect for the social sciences though I may have.