I don't know how you got that from the essay.
I believe Sniffnoy, like myself, gave the author the benefit of the doubt and assumed that he was not actually trying to argue against a fundamental principle of logic and decision theory but rather claiming that the principle applies to humans far less than often assumed. If this interpretation is not valid then it would suggest that the body of the post is outright false (and logically incoherent) rather than merely non-sequitur with respect to the title and implied conclusion.
Sniffnoy claims that gwern has argued "that the sunk cost fallacy is a mode of reasoning which doesn't often occur, rather than one that is valid."
Actually, what gwern has argued is that while the sunk cost fallacy is often used as an heuristic there is little evidence that it is sound to do so in real world situations. This also seems to be what you've said, but it is not what Sniffnoy has said.
Hence my confusion.
On a side note, I don't really understand your qualms with the title, but that's less important to me.
I just finished the first draft of my essay, "Are Sunk Costs Fallacies?"; there is still material I need to go through, but the bulk of the material is now there. The formatting is too gnarly to post here, so I ask everyone's forgiveness in clicking through.
To summarize:
(If any of that seems unlikely or absurd to you, click through. I've worked very hard to provide multiple citations where possible, and fulltext for practically everything.)
I started this a while ago; but Luke/SIAI paid for much of the work, and that motivation plus academic library access made this essay more comprehensive than it would have been and finished months in advance.