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.
Thanks for you comment, it makes a good point . My comment was hastily written and I think my argument that you're referring to is weak, but not as weak as you suggest.
At some points the author is specifically critiquing longtermism the philosophy (not what actual longtermists think and do) eg. when talking about genocide. It seems fine to switch between critiquing the movement and critiquing the philosophy, but I think it'd be better if the switch was made clear.
There are many longtermists that don't hold these views (eg. Will MacAskill is literally about to publish the book on longtermism and doesn't think we're at an especially influential time in history, and patient philanthropy gets taken seriously by lots of longtermists).
I'm also not sure that lots of longtermists (even of the Bostrom/hinge of history type) would agree that the quoted claim accurately represent their views
But, I do agree that some longtermists do think
And there's some criticisms you can make of that kind of ideology that are similar to the criticisms the author makes.