A long leash wouldn’t necessarily have stopped the dog from jumping off the bridge. The little guy could have snapped his neck.
Dogs also slip their leashes all the time.
That said, I don’t see her holding an empty leash in any of the pictures.
But, maybe she set it down in her panic.
I’m a guy in my mid-30s who is in really good shape, and I can’t swim for beans in choppy water.