“The last time they’d all met together to talk, they hadn’t yet been angels and demons. They’d all been together, on the same side. They’d all been spiders.” Um, what? Is this a metaphor? A typo? Or were angels and demons in this setting literally insectivorous webspinning arachnids prior to their falling out?

It’s sort of a metaphor! Prior to the current division of magic into five types that we see in the main story, all magic users in the world drew on a power source called the Web, and were therefore colloquially called spiders. Basically what Jude means when he says that is that back in the day they were all humans fighting in the same army. 🙂