The 5.3GHz value is the Thermal Velocity Boost:
However this value is not the typical turbo frequency (Max Boost 2.0) that was rumored thanks to numerous leaks, instead, this is a Thermal Velocity Boost, Intel’s yet another way of boosting the clock speed under specific conditions. In this case, the clock will reach 5.3 GHz if the temperature and the power budget allows. The TVB was first introduced with Core i9-8950HK, Intel’s first i9 for the mobile platform.
Via: VideoCardz