Gateway resource selector and gateway workload instance

Using Istio 1.2, based on https://istio.io/docs/reference/config/networking/v1alpha3/gateway/#Gateway

In other words, the Gateway resource must reside in the same namespace as the gateway workload instance.

the selector should select the ingress gateway pod in the same namespace.
However, deployed the bookinfo app in default namespace and created Gateway in default ns.
It’s still able to use the istio-ingressgateway under istio-system ns.

Is the doc wrong?