The dialog he got when he plugged it in mentioned FTDI and "Vinculum" which appears to be a programmable USB interface chip with serial and SPI support https://ftdichip.com/products/vnc1l-1a/ https://ftdichip.com/software-examples/vinculum-projects/

Since it's not just a generic non-programmable serial chip I assume it's also doing something more. But it doesn't do Ethernet so I bet it's not actually PoE but like... power+serial over Cat5

It's an FT312D, which is a purpose-built android thingy that presents a serial port and allows the tablet to charge.

Yeah given that USB doesn’t work over long distances there is probably a simple serial protocol waiting to be reverse engineered.