marxism

No problem, the author basically also mentions it in other words. Question-begging is a method of argument where you take the point that you are trying to "prove" and just accepting it as a premise instead of actually proving it (which can be done covertly or blatantly). As the author points out, even if they extend it more broadly than I would, a lot of "AES" discourse starts from the standpoint of basically assuming that a given party is trying in good faith and with solid competence to implement socialist policies, when usually it is the point of contention from the anti-revisionist crowd and others that they are not, so the question of what is and isn't socialism (and especially Marxism) becomes not a question that we can answer by assessing methodology and the like, but just a collection of inherited rulings. It can be given a further appearance of argument by throwing up some superfluous justifications (e.g. pointing to the difficulties faced by the party), but fundamentally it is hostile to the idea of having any sort of meaningful falsification criteria (e.g. it gives us little idea or a completely fanciful idea of how a non-socialist state would respond differently).

Obviously, there are many "AES" defenders on this instance and elsewhere who engage in much better faith than that and do try to assess methodology, etc., I'm just saying this is a thing some people do and you'll see it pretty often.