And cast lead bullets break up upon impact.
It's not impossible for a cast bullet to break up on impact, but it's certainly not a given. The main reason for using cast bullets is to minimize deformation and maximize penetration. Which they generally do pretty effectively within the range of velocities that they are intended to be used.
In which case, you will never be able to recognize what you've done, draw, aim and fire accurately enough with any hand gun for it to help you.
It is true that bears are much faster than people tend to expect them to be. However there are documented cases of people successfully killing attacking bears with handguns (including 9mms) so the idea that it's "never" possible is certainly incorrect.
I think it's a given that 9mm is not going to be a popular choice for bear defense, nor is it a caliber likely to be recommended by experts for that purpose. But with the right ammunition, some luck and a lot of skill, it will provide the defender a chance of survival.
Put it this way. If I were going to buy a handgun specifically for bear defense I wouldn't spend any time looking at 9mms. But if I needed a handgun for bear defense and I already had a 9mm that I shot very well, I wouldn't
automatically sell it to buy something in a different caliber.