In it, he shows an example of using all three ESP32 UARTs from an Arduino program. He calls the third port “secret” although that’s really a misnomer.

The write code also can detect an I/O port write and sends that data to the Arduino serial port. It doesn’t appear to matter what I/O port you write to.

It's of course backwards compatible with existing Arduino add-ons, but sets itself apart with a new USB controller with lower latency and driver-free operation on Mac and Linux.