Ubisoft says it was trying to put an ad for Assassin’s Creed Mirage in the main menu of other Assassin’s Creed games. However, a “technical error” caused the promotion to show up on in-game menus instead.
There’s nothing accidental (or “accidental”) about the ad being in the game. Probably was an accident that it appeared mid-game, they need to desensitise you to ads in the menus first.