Acceptable loss of crew event is like 1 in 250 (can't remember the exact number). They can't quantify the probability of failure, so not putting them in Starliner is the right call.
For the journey home from ISS to Earth, the probability threshold is 1-in-1000. Likewise, it is 1-in-1000 for the journey from Earth to ISS.
The riskiest part, which increases the probability from 1-in-500 to 1-in-270, is the ISS stay – the extended stay in space is faced with a continuous risk of micrometeoroid damage.