AOC is a bourgeois politician. There is nothing she can do that is right, unless you are a social democrat who want to improve conditions of the working class so that there is less class conflict. Whatever social democrats can do to improve things, actual communists can do better through independent working class power. Read the section on bourgeois socialism in the Manifesto, that explains why we should be opposed to such movements.
What is sectarianism exactly? I still don't get it. Like disagreements actually exists in the real world. Why do you think people talk about left unity all the time? I would define sectarianism as letting disagreements get in the way of real world action. So sectarianism is a real life problem. In the online world, I don't get what uncriticial left unity, and silence for the sake of unity, would achieve.
Why would saying true things be sectarian? If something is true, why shouldnt it be said? Is it that these things hurt their feelings? By this logic, claiming that USSR is socialist is sectarian, because it hurts my feelings, regardless of whether it is true or not.