Ext_authz server_uri.uri for http_service

Today, ext_authz http_service filter configuration requires the property server_uri.uri to be filled but it does not seem to be used although envoy will fail to boot up if it does not have at least the string "http". I was expecting that the filter would at least use that information to connect to the correct upstream authentication service and send the correct hostname for my load balancer but instead it seems to only use the cluster information and then it sends the original host in this request. Is this supposition correct? Or am I missing something?