It's been a year, but I finally wrote up my critique of "longtermism" (of the Bostrom / Toby Ord variety) in some detail. I explain why this ideology could be extremely dangerous -- a claim that, it seems, some others in the community have picked up on recently (which is very encouraging). The book is on Medium here and PDF/EPUB versions can be downloaded here.
I haven't read the top-level post (thanks for summarising!); but in general, I think this is a weak counterargument. If most people in a movement (or academic field, or political party, etc) holds a rare belief X, it's perfectly fair to criticise the movement for believing X. If the movement claims that X isn't a necessary part of their ideology, it's polite for a critic to note that X isn't necessarily endorsed as the stated ideology, but it's important that their critique of the movement is still taken seriously. Otherwise, any movement can choose a definition that avoids mentioning the most objectionable part of their ideology without changing their beliefs or actions. (Similar to the motte-and-bailey fallacy). In this case, the author seems to be directly worried about longtermists' beliefs and actions; he isn't just disputing the philosophy.