If you read The Scar it goes into how strange the setting is and how broken reality is in places. Some of the things in there may or may not explain some of the races, lime the cactus people. However, it is all kept very vague.

You should probably take it as a reaction against Tolkeinesque fantasy that draws on northern European myths. There's a wide range of influences as well as Mieville just making up oddities just for the sake of it. So, for example, Kephri is an Egyptian scarab-headed god and then everything is built around that. It doesn't have to be plausible it just has to contribute to the strangeness.