NASA wanted to understand if the malfunctioning thrusters could operate within a margin error for safety. Boeing spent a month running a study that produced non-definitive answer as to the root cause of the problem. Had Boeing definitively demonstrated the root cause of the thruster issue NASA probably would have sent the Astronauts home on Starliner. It’s not that NASA necessarily thought that Starliner was unsafe. It’s more like they lack confidence that Boeing actually understands the problem.