Neoreaction is wrong, and is in fact speaking nonsense, when it says that Leftism is a Christian heresy.
What they should be saying is that Leftism is Christian apostasy.
Apostasy means to abandon the faith; whereas heresy means contrary to orthodoxy. That clarification should be enough to show the error of Neoreaction.
Heresy can only be defined from one particular concept of Christian orthodoxy - so a Roman Catholic, a Russian Orthodox or a Calvinist could define some other person or group as heretical - indeed each would define the others as heretical - but strictly there are not generically Christian heretics, because there is no generically Christian orthodoxy from which to define heresy.
If somebody dissents from a generically Christian position, then they are not an heretic, they are an apostate - which is to say they are not merely unorthodox Christians but reject Christianity itself (for example, if they deny that Christ is their savior and Lord, but merely a prophet, teacher or good example).
Some Liberal Christians, who are always Leftist, could be described as heretics from the perspective of other specific Christian positions - but some are apostate since they deny basic, core Christianity.
Neoreactionaries logically cannot define, detect nor ascribe heresy, nor can they describe some person or group as heretical, because they are not arguing from any orthodox Christian position; they are in fact (qua Neoreactionary, and even if they themselves happen to be Christians) arguing from a secular position.
However, although Neoreaction cannot ascribe heresy, because it is neither Christian nor orthodox in any way, Neoreaction can legitimately detect and define apostasy: it can recognize that some person or group has abandoned core Christianity.
Neoreactionaries as individuals can only define heresy if they are doing so from a specific orthodox Christian position - but this will not merely relate to Leftism, but a Calvinist Neoreactionary will then define a Roman Catholic Neoreactionary as heretical and vice versa.
Anyway, enough has been said to demonstrate that the Neoreactionaries 'Leftism is a Christian heresy' meme is not just causally incorrect, it is logically incoherent.
Not just nonsense, but double nonsense!