The treated mean is taken at face value as E[y(1)] and nothing at
all is assumed about E[y(0)] beyond the observed support, so the
counterfactual mean is placed at each end of that support in turn. The
width of the interval is exactly the range of y, whatever the data
look like, which is the point: the bound shows how little the data alone
deliver.
References
Manski, C. F. (1990). Nonparametric bounds on treatment effects. American Economic Review Papers and Proceedings 80(2), 319-323. Restated as equation (2.11) of Molinari, F. (2021), Microeconometrics with partial identification, Handbook of Econometrics 7A, 355-486 (arXiv:2004.11751 p. 17).
