Difficult to judge without a description of the weather conditions in more detail than “thunderstorms”. Thunderstorms can be extensive or small, slow or fast moving, isolated or frequent. If they were small, fast moving and isolated (‘showery’ type) then the crew would have banked on being able to hold off for a few minutes if needed to let a storm pass. If extensive and slow moving (‘frontal’ type) then it might not have been such a good idea. But another feature of thunderstorms is that they can develop and fizzle out rapidly, so provided you have a guaranteed good weather diversion it might still be judged worth an attempt. Things can change a lot even in the space of a 20 minute flight; it just seems not to have worked out on this occasion.