A student of the Tashbar on the sanctity of the seventh in a foreign crop

I accidentally deleted the question. I'm reposting it here:
I grew up in Bnei Brak and studied in the Talmud Torah in the Beit Chazo\"a [Tishber] when it was the year of Shemitah. Some of the students practiced Kedushat Shevi'it with leftover fruit and vegetables even though they came from non-Jewish crops. Some did not. When I inquired, I was taught that Kedushat Shevi'it does not practice non-Jewish crops except for the Chazonishniks who practice it [a few percent of the population] and a few Ba'al Teshuvah [whose tendency to be strict is understandable…] Similarly, in my parents' house and the neighbors did not practice this strictness. Now I saw a book by a respected Rabbi Srog who writes that indeed there is no need to practice the practice of the seventh-day holy harvest of Gentiles except in Bnei Brak, where the custom of the prophet "A" was accepted to practice the practice of the seventh-day holy harvest. Therefore, and accordingly, in Bnei Brak, this is how it should be practiced. That rabbi is mistaken in reality. Yemenite Sephardim and Hasidim of course do not practice this way in Bnei Brak [and they are the majority]. The Lithuanians in reality on the ground, only a minority of them actually practice this way. Is therefore the halakha different from what that rabbi wrote? [He is mistaken in reality]