I don't think he means controlling the faces II base itself, I'm sure he means you can communicate with a module that uses serial comms mounted on the base using a grove cable from a separate m5stack core (with a serial port or pins).
The grove port on the m5stack will supply the power for the module on the base and serial tx/rx data will be transferred over the cable instead of directly over the internal M-BUS. Which is how it will usually work with a stack of modules under a m5stack core.
I'm not sure if this "remote" serial cabled comms is actually very useful in the real world.
Edit: I just realised that the faces finger board uses serial (uart), so you could put a fingerprint faces board on the faces base and communicate with it over blue port c using pins 16/17 from a separate m5stack core.