On the Pinnacle 100 MQTT is implemented in the Zephyr IP stack. The MQTT protocol managed by IP stack in Zephyr running on the nRF52840. The MQTT layer uses sockets in order to send and receive data. The HL7800 driver in Zephyr does all the work to expose socket APIs to be used by a user app or other software layers.
This approach provides a lot of flexibility. The Pinnacle 100 supports anything that the Zephyr IP stack supports.