I mean, Soyuz is the emergency rescue plan. Usually 2 of them sit docked to the ISS at all times for just this purpose. But regardless, the Starliner is functional enough they could leave right now if they had to. They just aren’t stranded, NASA isn’t just like… lying.
The Starliner has redundant systems and even with several thrusters offline it’s still within safe operating parameters. They’re keeping it docked because they want to figure out the problem, not because they need to figure out the problem.
There isn’t a dragon capsule ready to go at the moment, but it doesn’t really matter, it shouldn’t be needed. Because as I said, nobody is stranded, at least not yet.