That bit reads to me as just a heading of one section of the article - a paragraph later it lays out the argument which is described as being "pretty air-tight". Which argument does assume one has a particular kind of ethical system, but that's not really the same thing as making the confusion you describe, especially when it's an ethical system shared and trumpeted by many in the community.
Which argument does assume one has a particular kind of ethical system, but that's not really the same thing as making the confusion you describe
Under this logic I can easily say "the intellectual case for killing infidels is pretty air-tight" or "the intellectual case for torturing suspects is pretty air-tight" because hey, we abstracted the values away!
I'm currently unconvinced either way on this matter. However, enough arguments have been raised that I think this is worth the time of every reader to think a good deal about.
http://nothingismere.com/2014/11/12/inhuman-altruism-inferential-gap-or-motivational-gap/