Completely untrue, there are currently only 3 human-rated spacecraft docked to the ISS and none of them are set aside as some sort of emergency capsule. There’s no trickery here. The number of astronauts on board is equal to the number of seats available for them to ride back home in. They aren’t stranded only because Starliner is still fully capable of undocking and taking them home. If it wasn’t, then they would actually be stranded with no alternative way back beyond straping them to the floor of Dragon.