This folder contains the compiled files, not all the source code and other files. Also note that the latest nordic-nrf52 platform version uses Zephyr 2.2.0 release version so it’s not identical to the bleeding edge github version you linked to.
The ST7789 driver is present in the Zephyr framework bundled with the latest PlatformIO version.
Enabling it seems to be done via the Zephyr prj.conf file. Refer the example platform-nordicnrf52/examples/zephyr-blink at develop · platformio/platform-nordicnrf52 · GitHub and the relevant KConfig files in the driver code ( CONFIG_DISPLAY, CONFIG_ST7789V and friends).
PlatformIO has documentation available at Zephyr — PlatformIO latest documentation for configuration and pulling in external Zephyr modules.