I’ll give it a shot….
Do you think the CC held that Rabbeinu Tam and his kehilla were mechallel Shabbos?
Maybe (but only maybe). The CC also held the same for the Gaon, who (might have) done melocha 13.5 (IIRC) minutes after the sun is no longer visible (early shkiyah). Therefore, the CC recommends that one be Machmir for both shittos. That is not to say that holding of either shittah makes one “a shaigets or even has no halachic grounds to stand on”. In fact, there are quite a few practical Halachic Nafkei Mina during the “in-between” times that the CC would agree to since the Z’man is only a Safek.
That being said, the Biur Halacha does say one should keep “early” shkiyah, and is quite explicit about it.