If the current price $P_{t-1}$ is below the target price $E[P_T]$ is the drift not $\theta(P_{t-1}-E[P_T])$ (negative) ? Seems to me the authors have a sign error. Regarding the possiblity to overshoot. In a OU process this is usually achieved by the stochastic term which is $\epsilon$ in your case. In short: the expected process is pulled towards the long ...