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!
Well, yeah, if you have an essay about infidel-killing, having the subheading for the part where you lay out the case for doing so describe said case as "pretty air-tight" isn't exactly a heinous offence.
And you're kind of skipping over considerations of what values Less Wrong tends to have. There's a lot of effective altruism material, members of the community are disproportionately consequentialist, are you expecting little asides throughout the article saying "of course, this doesn't apply to the 10% of you who are egoists"?
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/