The listen address is the one that alertmanager binds to. Generally, this can be 0.0.0.0 (all interfaces), I believe that is the default and thus sometimes omitted.
It will try to guess which address it can be reached at by other AMs (which address it should advertise to them). In some circumstances this doesn't work right. A Kubernetes pod probably isn't one of those circumstances, but specifying the address explicitly doesn't hurt either.
So in short, I would expect everything to work out of the box in a standard Kubernetes pod, but you can be more explicit if you want to be.
/MR