Right, not doing that is the part where he let them go. Basically, he clearly didn't intend to let Ezra and Kanan go, but when they managed to knock him down and get to the shuttle, he chose to let the shuttle go because tracking down the rest of the cell was more important.And there is that lovely shot from Kenobi, where he pulls a ship that just launched back to the ground with the force, then force-tears it's hatch open.