That's not very charitable. :-P

Mackie:

Even more important [than the argument from relativity], however, and certainly more generally applicable, is the argument from queerness. This has two parts, one metaphysical, the other epistemological. If there were objective values, then they would be entities or qualities or relations of a very strange sort, utterly different from anything else in the universe. Correspondingly, if we were aware of them, it would have to be by some special faculty of moral perception or intuition, utterly different from our ordinary ways of knowing everything else. These points were recognized by Moore when he spoke of non-natural qualities, and by the intuitionists in their talk about a 'faculty of moral intuition'. Intuitionism has long been out of favour, and it is indeed easy to point out its implausibilities. What is not so often stressed, but is more important, is that the central thesis of intuitionism is one to which any objectivist view of values is in the end committed: intuitionism merely makes unpalatably plain what other forms of objectivism wrap up. (Ethics: Inventing Right and Wrong, p. 38)

I think Seán's take is pretty fair, if a bit facetious. Consider what Mackie takes to be a significant concession:

Indeed, the best move for the moral objectivist is not to evade this issue, but to look for companions in guilt. For example, Richard Price argues that it is not moral knowledge alone that such an empiricism as those of Locke and Hume is unable to account for, but also our knowledge and even our ideas of essence, number, identity, diversity, solidity, inertia, substance, the necessary existence and infinite extension of time and space, necessity and possibility in general, power, and causation. If the understanding, which Price defines as the faculty within us that discerns truth, is also a source of new simple ideas of so many other sorts, may it not also be a power of immediately perceiving right and wrong, which yet are real characters of actions?

This is an important counter to the argument from queerness. The only adequate reply to it would be to show how, on empiricist foundations, we can construct an account of the ideas and beliefs and knowledge that we have of all these matters. I cannot even begin to do that here, though I have undertaken some parts of the task elsewhere. I can only state my belief that satisfactory accounts of most of these can be given in empirical terms. If some supposed metaphysical necessities or essences resist such treatment, then they too should be included, along with objective values, among the targets of the argument from queerness. (p. 39)

Basically: Naturalism says ethical facts would be queer. We should be naturalists, so we should not think that there are ethical facts.

Well, without Seán's humorous syllogistic formulation, under philosophical naturalism (or more broadly any metaphysical system that accepts the fact/value distinction) Mackie would be correct that morality is a "queer" property.