scipy.stats._continuous_distns.chi2
https://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.chi2.html
scipy.stats.distributions.chi2
でアクセスしているのはこれ
sf(x, df, loc=0, scale=1)
Survival function (also defined as 1 - cdf, but sf is sometimes more accurate).
https://github.com/scipy/scipy/blob/v1.9.3/scipy/stats/_continuous_distns.py#L1433
分布の実装は
chi2_gen
https://github.com/scipy/scipy/blob/v1.9.3/scipy/stats/_continuous_distns.py#L1366-L1430