The limits are found by numerical integration over Lindley's density.
References
Lindley, D.V. (1988) Statistical Inference Concerning Hardy-Weinberg Equilibrium. In:
Bernardo, J.M., DeGroot, M.H., Lindley, D.V. and Smith, A.F.M. Bayesian Statistics, 3,
pp. 307-326. Oxford University Press.
Graffelman, J. (2020) Statistical tests for the Hardy-Weinberg equilibrium.
Wiley StatsRef: Statistics Reference Online tools:::Rd_expr_doi("10.1002/9781118445112.stat08274").