Stranded: Left without the means to move from somewhere.
If you are stranded on an island, it implies you have no means of return. They have a means, to return on the same capsule. They also could return on another capsule. NASA decided to not take the risk, as there are other less risky options available.
If there were no other options, NASA would send the back on Starliner, and it would still likely be much safer than something like the space shuttle
Nobody plays these semantic games with the word stranded in any context other than Starliner. If somebody's car breaks down they can fairly claim they were stranded even though they could call an uber or even hitchhike and nobody pisses and moans about them not truly being stranded according to some dictionary definition of the word. Boeing has made you their PR bitch.
If you are stranded on an island, it implies you have no means of return. They have a means, to return on the same capsule. They also could return on another capsule. NASA decided to not take the risk, as there are other less risky options available.
If there were no other options, NASA would send the back on Starliner, and it would still likely be much safer than something like the space shuttle