There's only so much maneuvering space, and so much time a single brushing session takes; perhaps the error of double integration is small enough they can get away with dead reckoning. Especially since you mouth is a small, confined space, so they could estimate its bounds and use that to further correct their position estimate.

(I imagine a system like this would need at least two accelerometers - one close to the tip, and another one in the grip area; the second one would be used to cancel out the movement of users' head and body, allowing the first one to effectively operate in a mouth-relative coordinate system.)