“We are talking technically. And not in the Zman HaBayis.”
During the time of the Sanhedrin the Beis Din could enforce their authority. But even in Europe for many centuries the local governments gave the Jewish community autonomy to enforce halacha. Declaring oneself non-frum/secular did not prevent Beis Din from asserting jurisdiction over him. Even in Europe and in the Arab countries. We would do so today in America as well if the goyim didn’t prevent us.