It's 'compatible' in that they can do the work to integrate it with another rocket if needed, which isn't really saying much because most payloads are like that

It's 'compatible' in that they can do the work to integrate it with another rocket if needed, which isn't really saying much because most payloads are like that. It isn't compatible with Falcon 9 in its current state, and IIRC because it's wider than F9, actually flying Starliner on F9 would require a lot more structural work too (devising an appropriate aerodynamic adapter and ensuring structural loads are acceptable). Plus, NASA crew rates the full stack rather than treating the rocket and capsule separately, so integrating Starliner on another rocket would require the crew rating process to be repeated (granted, it'd be a bit easier since F9+Dragon is already crew rated).