I want to be careful to say that I don't think calling non-Catholics heretics on Twitter is an effective or Christlike approach, but as far as I know, Marshall is technically correct.
Technically speaking he is incorrect, but let's pretend he said something more precise: "If you are baptized and you deny any of these four dogmas you are a heretic and anathema..." Would he be technically correct in this case? If he limited his statement to the baptized?
I think not, and this is especially true given (4), but it depends somewhat on what he means by 'heretic' and 'anathema'. Heresy generally involves a falling away, and anathema generally involves some form of excommunication. But someone who is not part of something cannot fall away from it, and someone who is not in communion cannot be excommunicated. Therefore using the words in such a way would seem to be nonsensical, and all of this is particularly true as regards the dogma of the Assumption. It is like telling a Hindu that they are at fault because they do not believe a Catholic dogma on the authority of the Church, or trying to send someone to prison who is already in prison.* The statement is only plausible with respect to Catholics.
Of course one can plausibly predicate heresy or anathema of historical groups that span many generations, albeit in a secondary sense. If Marshall was saying that such people are branches of heretical roots and therefore heretical then this could be appropriate, but I really don't think that is what he was saying.
Those who aren't Roman Catholic are already essentially heretics.
Exactly. Whether or not one thinks of non-Catholics as heretics, there is no change of status occurring, and therefore the charge is vacuous.
* Specifically, "heretic" connotes fault. "Heresy" or "heretical opinion" usually also connote fault, but they can be used in a way that prescinds from culpability. I would only consider granting Marshall the technicality if he had used one of these two terms and omitted "anathema," but given Marshall's nature it would still be unintuitive to interpret him in such a way.