Both Tessel 2 and WrtNode use 3.3V levels for serial and all other IO.
The nonstandard USB cable included with WrtNode only supplies power to the board, and does not connect data to the male A plug. The data and power pins are connected to a USB A female port spliced inline in the cable where you can plug in a USB device that will be visible to the Linux system on the board.
Tessel 2 has a normal MicroUSB port to accept power and connect to a computer. That USB connector provides a USB-serial interface along with a data channel used for Tessel CLI commands. No external serial adapter is required with Tessel 2 hardware. It also includes two USB A female ports for connecting USB devices.
P.S, the blog post incorrectly calls the MT7620n development board an "OpenWRT Node". It is actually not affiliated with the OpenWrt project and is called WrtNode.