Order books will almost always be single threaded or something similar. Only one order action is allowed at a time in each book, otherwise it would be chaos. Exchanges are very strict on "fairness" since if there is a (legal) edge to be had, you can bet that someone will find it and try and exploit it. That's just the game.
I really wonder here, in this part of "Introduction to Limit Orders", how they handle race conditions. Do you think they use a synchronous queue that queues orders while it's transacting, or do they try to also handle inserting orders during the process of filling orders?