I'd argue again that a lot of that has more to do with longtermism being a pretty PR-savvy movement than with any actual philosophical aversion. That is to say, I don't know how meaningful an argument it is to say they don't explicitly describe themselves as "pro-genocide" — even a lot of people actively perpetrating genocides wouldn't advertise themselves as genocide advocates. It's a pretty negative term that virtually nobody goes out of their way to be associated with. The problem is not that longtermists go around saying "We really ought to do more genociding," it's that longtermists go around arguing that essentially any present-day sacrifice short of human extinction is trivially easy to justify according to their deeply wonky assumptions about the future.

Which brings us to the next point. As you say, this is the kind of thing that has always posed a problem for utilitarianism, but I'd argue that there is a salient difference in the case of longtermism simply because the its ethical calculus is predicated on the kind of abject loonytoons gibberish that has to be forced onto utilitarians. In other words, while it may be possible to argue against both with "weird, unrealistic hypotheticals," the difference is that longtermism is specifically built around those very hypotheticals; the whole "digital people" thing from my prior post wasn't some weird thought I experiment I made up to put longtermists in a bind, it is a thing longtermists themselves bring up as part of their arguments.

And, to your final point, I'd generally agree, but I think it also encapsulates what's wrong with longtermism: it takes the implications of its "weird, hypothetical thought experiments" very seriously. Premises that others would hesitate to accept even for the sake of a purely theoretical or counterfactual debate, longtermists often treat as concrete assumptions on which to base our actual day-to-day ethical decisions. If you don't take the hypotheticals seriously, you can't take longtermism seriously. If you do take them seriously, longtermism is horrifying.

The only parts of longtermism that aren't predicated on dubious assumptions are the parts predicated on absolutely bonkers assumptions. The core idea sounds reasonable enough, but once you start digging into the actual arguments, it quickly devolves into clownshoes nonsense. The public face of the movement is the banal and agreeable assertion that it's important to care about future generations, then the actual ideological content of the movement is largely stuff like arguing it's okay to use slavery and genocide today to help pave the way for something like a galaxy-spanning network of planet-sized servers that host a population of trillions upon trillions of "digital people" living in simulated utopias. It's basically just the philosophy of using faintly ludicrous assumptions about the future as an excuse not to care about the suffering caused by our actions today.