"Once the story gets to the point where it introduces giant alien snails the size of mountains from other planets, it's science fantasy."
I couldn't disagree more on this point. If anything, the idea of aliens as truly different, in size and structure and purpose, makes it more credable sci-fi to me. How many stories and movies depict aliens as totally anthropomorphic (klingons, vulcans, asari, minbari, etc.) and eating the same food as us, having sex with us, walking around on two legs and wearing clothes and basically being exactly like us?
I found the shell creatures in this story to be mind-blowingly unique, like something that is perhaps ordiniary on its own world, but displaced to Earth through some kind of explosion or collision or whatever, makes them monsters here. Like ants from Earth are just a nuisance here, but if they somehow hitched a ride in a starship and ended up on an alien world, it might be totally monstrous and terrifying the inhabitants there. Anyway, great story and something really different, choosing an entirely original setting (ancient Persia!) with a truly unique alien and the downright nightmare-inducing way that humanity is forced to adapt. That's what sci-fi is supposed to do, instead of just introducing another Deep Space 9 habitat for humans and human-like aliens to chat around a common tavern area.