Citizen’s United is irrelevant. This isn't an issue of whether or not Twitter is a person protected by the 14th Amendment, but whether it is the federal or state government, and thuse restricted by the 1st and/or 14th amendments. And–as I would think would be obvious–Twitter is neither the federal government nor a state government. (And, if you wanted to make the argument that it was an agency of state government because corporations are created by states through law, that would be more interesting, but then the "private company" vs. "public company" issue would still be moot, because that would apply to all corporations regardless of whether their stock was publicly traded.)

> I can’t wrap my head around this notion that an agent of $40B public company constitutes “private censorship” do some research.

I have an undergraduate degree and some professional study (abandoned midway for an IT career) in this field. What you are doing is called “equivocation”; making an argument that attaches the import of one meaning of a word to a circumstance in which a different meaning applies. A “public company” in the sense that Twitter used to be one is a short way of saying “A private corporation with stock traded on a public stock exchange”.

It is not the same thing as a government entity subject to the 1st (federal) or 14th (state, and consequently subdivisions thereof) amendments to the US Constitution.