1000km/s on the energy-limited Orion would be hard to slow down using aerobraking without potentially disrupting the planet below, and reverse slingshots would probably occur too fast to do anything more than staying in-system for a few weeks...
It's not that slowing down is impossible...you just start throwing bombs the other direction. But by slowing down you increase your travel time, so the 150 year estimate is no longer valid.