The RTE is accountable for the ART’s effectiveness.
The RTE helps the ART to be more effective by enabling Agile Teams to improve its practices, within the SAFe framework. Through working together using SAFe, inspecting progress and adapting their plans, the whole ART assurse delivery is low risk.
As the RTE isn’t accountable for the work itself, they’re not responsible for delivery in the same way a Delivery Manager or Project Manager is responsible for delivery.