I guarantee you these books knew Brexit probability was higher then it was trading. Public money was pouring in on remain though so no need to adjust the odds when your making out like a bandit.
I disagree here, they don't "know" shit, the good news is neither does anyone else, this is
likely the biggest misconception, everyone is in the "know" but you, does anyone know where the spuz will close today, no proof either way
I'm not saying they had any insider information but books carry risk all the time when the public money is taking the other side. They were dealing 80% chance of remain when the correct odds may have been 60%.
When bets get really one sided they are more eager to lay it off. Where they get lucky is when they are only marginally one sided this can happen when "favorites" get overbid. For example, the pats are routinely over bid throughout the season and sports books collect on that, think of it like skew on an option. Teams like the Packers and Cowboys also get routinely over bid. So they earn a little yield on that. But when something is completely one sided, they usually will not put their solvency at risk to take a spec bet.