I wouldn't be surprised if it's ethernet after all. Ethernet over Thunderbolt achieves multiple gigabits, enough for uncompressed 1080p@60Hz.
For reference, DisplayLink DL-5xxx has been doing 4k over USB since 2014. https://en.wikipedia.org/wiki/DisplayLink#DL-5xxx_(2014)
Linux has had the pieces around for a while. Usb-device-mide gadgets in Linux can be connected to a usb-ip virtual port, so you can share virtual peripherals. The rest is regular networking. Wild but USB gadget mode has been around here and there in laptops for a long long time now, but has rarely ever been used. Here's a steamdeck acting as USB printer, https://hackaday.com/2023/01/31/steamdeck-become-printer/
Getting an add-in card that can do usb-device-mode / usb-client mode can be tricky but is possible. This capability seems standard on most single-board-computers like RPi (since rpi1 iirc?). https://github.com/ufrisk/pcileech/issues/21
There is support in gstreamer. The example shows piping the standard test signal out as uvc gadget. https://gstreamer.freedesktop.org/documentation/uvcgadget/in...
There's various write-ups out there for example using a RPi as a USB webcam, using this. These write-ups cover more of the details of - as an end user - setting up/configuring the gadget device. https://github.com/geerlingguy/pi-webcam?tab=readme-ov-file#...
So I bought a couple 2.5G USB 3 adapters and confirmed: iperf3 reports 2.5 gigabit between my laptop and desktop connected to the same switch.
Much cheaper than getting a new NIC for the desktop.
gigabit Ethernet over a switch under my desk makes nomachine almost pleasant.
If you mean RJ45 built into the device then I don't see how that's implied by the above comments.