The Helm install of Rancher uses Traefik by default. Does the Istio Helm chart replace it, or do I need to specifically attach it to a free IP address to use Istio as a replacement ingress?

The helm install of Rancher uses whatever ingress you have available on your cluster, it does not include an ingress controller. You can select the Ingress class for it to use via a chart value if you for some reason have multiple installed and want to pick.

Thanks. I thought it installed Traefik, since the install was on a nearly fresh system, but perhaps it was added by the k3s setup instead and I missed it (which is probably pretty likely).

Yes, k3s does come with traefik