Huawei states that its Mate 20 X features Huawei Supercool - "the world’s first liquid multi-dimensional cooling system with Vapor Chamber (VC) and Graphene Film used in smartphones". It reportedly conducts heat away fast, keeping the device cool even under heavy loads (as the device is touted as a gaming device).
These news, while exciting, do not come as a complete surprise - Huawei has been investing heavily in graphene research and in November 2016, Huawei introduced new high-temperature Li-Ion batteries that use graphene to reduce operation temperature and increase the lifetime.