Epicureans were right about the “swerve”. It is called random or chaotic behavior in modern science. It is found in a lot of systems, like the wheater. Importantly, the mutations that are at the core of biological evolution are random events, which causes evolution to be a contingent process, non-deterministic. It takes place between constraints, but it is theoretically impossible to predict the outcomes of future evolution. This has been explained in detail by Stephen Jay Gould and Stuart Kaufman (in At Home in the Universe).
Causation is not the same as determinism because there are causal loops: A causes B, B causes C, C causes A. If there is positive feedback in such causal loops, the system gets out of control and it is impossible to predict its behavior. It is a causal, non-deterministic process. There are lots of those in the world, which is why the world we live in is contingent and non-deterministic. For example, take a pencil, balance it on its point and let it go. This is a non-deterministic event because one can not predict in which direction the pencil will fall. Small changes in the initial conditions get amplified to produce larges differences in the final outcome. We, humans, are non-deterministic systems as well.
Therefore, the metaphysics of the Epicureans was right. The Stoics were wrong in their fatalism because the world is non-deterministic.