Complex sentence: To clarify, my opinion is that it is the specific theories advanced in that one particular Twitter post that are; fringe-ness, at least under our policy, applies to theories and ideas, not people.
Split into simpler sentences: To clarify, I am not saying that "you" are fringe; my opinion is that the specific theories you advanced in that one particular Twitter post are. Fringe-ness, at least under our policy, applies to theories and ideas, not people.