That is the definition of being stranded. They wouldn't have stayed back if they could have gone with the vehicle.
So even though everyone fits in the existing modules there is no spare if they let these two go home.
Boeing has stranded these astronauts on the ISS because it has no (acceptable) means of bringing them down, and the astronauts themselves have no personal means for doing so. If they want to become un-stranded then somebody will need to arrange the means for them to come down.
That doesn't mean that it's impossible for somebody else to provide those means, or that the means simply don't exist in any capacity. But they meet both the dictionary and common colloquial definition of stranded.