It's a software configuration as I understand it. The software itself is capable of the automated undocking, but it will need to be reconfigured to allow it.

ISS operations have very strict requirements about safety and especially about avoiding collisions with the station under any circumstance. There are also differences in requirements for crewed and uncrewed flights. For these reasons it makes sense that the configurations are different and would need to be updated if they switch to fully automated.

NASA has been pretty clear that Starliner could be used as an emergency escape if necessary. That leads me to think the concern is more about collision with the ISS that with the ability to re-enter safely.