This is a pretty interesting question. I have a ton of experience with sports betting, where money bet and by whom has no affect on the outcome of an independent event, at least in theory. I also have a little, by comparison, with financial markets where money bet actually drives the outcome. However, I have zero experience where the actions of all participants has an equal affect on the outcome, and they are also simultaneously betting on the outcome.
I haven't seen any volume numbers on the sides from the big bookmakers, but I assume they made money. The suckers (dumb money) were they guys, no matter how rich and smart they are, who were willing to lay 4 for 2 on a 50/50 prop where polling was inside the margin of error. The end result, known only in hindsight, is that the bookmakers got them to take a bad price, and the long odds they offered on Exit weren't enough to offset their profits on the Remain. They did a great job, people need to stop relying on betting markets, which has become fashionable among the "half sharp", for making predictions. They aren't trying to predict anything. They are trying to set a price that strips Expected Value out of their offerings, and puts the player in a long term negative position. They succeed and fail in this endeavor a lot, but their is no predictive magic in the pricing process.