The source for the fact that not-Frum people are considered Tinok Shenishba is in the Chazon Ish, Siman 2, S’eif 28. He also quotes the Chafetz Chaim as saying that all not-Frum people today are tinok shenishba.
In the Chazon Ish quoted above, he specifically quotes the Chofetz Chaim in Ahavas Chesed as saying that the concept of hating apikorsum does not apply today since they are “anusim” (acting out of coercion/tinok shenishba) and we are suppposed to sustain them and even be mechalel Shabbos to save their lives.