Don’t bring a sledgehammer to a handbag fight.
LOL.
I find it difficult to believe the robbers hadn't gotten through the glass by the time she arrived. But in any case, I notice that the security gates were descending from above, and I think the robbers saw that also. I'll bet that they realized that even if they broke in, they'd never get back out, and -that- is why they split. The granny did a great job of distracting them, though.