I would argue that, even using Stirner's definition, deliberate amorality and/or the rejection of moral judgement is still a moral system that guides a person's thoughts and behaviors, and so the whole post-left critique of morality is rather a moot point that, I would agree, is based on a narrow understanding of the concept.
I think your link provides the best critique when it says that:
"Stirner is clearly committed to the non-nihilistic view that certain kinds of character and modes of behaviour (namely autonomous individuals and actions) are to be valued above all others. His conception of morality is, in this respect, a narrow one, and his rejection of the legitimacy of moral claims is not to be confused with a denial of the propriety of all normative judgement. There is, as a result, no inconsistency in Stirner's frequent use of an explicitly evaluative vocabulary, as when, for example, he praises the egoist for having the ‘courage’ (265) to lie, or condemns the ‘weakness’ (197) of the individual who succumbs to pressure from his family."
In essence, he rejects all morality except the form of morality that fits his definition (which is still morality, despite his slapping a different title on it) and then he freely uses his own idea of morality to pass judgement on others, the same type of behavior that his "absence of morality" was supposed to get rid of.
Overall, the problem with amorality is that it is, like much of the rest of post-leftist thinking, philosophically sound (And what isn't, if you make up enough definitions and invent your own system of thinking to justify it?) but not realistic. People will never be free of morality, and it is naive to assume that is a possibility. Stirner's own adoption of what is essentially a moral system with a different name immediately after his rejection of moral systems stands testament to that.
And even if humans could be free of morality, then they would have to face the fact that, as Saul Newman said, the absence of moral judgement makes it impossible to develop of a critique of authority. Why become an anarchist if opression is not morally objectionable to you?