The reported value is
$$\hat\gamma_1^{(2)} = \frac{n}{(n-1)(n-2)}\sum_{i=1}^{n}\left(\frac{x_i - \bar{x}}{s}\right)^3,$$
where \(s\) is the (divisor-\(n-1\)) sample standard deviation. This
is sometimes called the “Type 2” or SAS/SPSS-default form; it is
approximately unbiased under normality.
Why isn't this in base R? R Core has historically deferred
higher-order moment statistics to contributed packages, in part because
three popular formulas exist (biased Type 1, bias-corrected Type 2, and
Minitab Type 3) and choosing a default would be opinionated. DMAR
adopts Type 2, which is the form most often used in psychometric
reporting and the one already used internally by
descriptives.
Diagnostic interpretation. As a rough rule of thumb,
\(|\mathrm{skewness}| > 2\) is sometimes flagged as indicative of
departures from normality large enough to threaten normal-theory
inference (e.g., maximum likelihood estimation in factor analysis or
structural equation modeling).