The UART bus exposed on the UEXT header is the same than the RK3399 SoC serial console exposed on the USB Type-C. There is no dedicated UART bus on the UEXT connector. The intention is to allow user to connect to SoC serial console by another way than the USB-to-Serial bridge.
!!! Important
If you connect some modules which might require dedicated serial UART line to the SoC, this module will not operate properly.