For the Wifi device mentioned in the original article, they dissasembled the firmware and found out that it uses "ThreadX": https://en.wikipedia.org/wiki/ThreadX

> "hreadX provides priority-based, preemptive scheduling, fast interrupt response, memory management, interthread communication, mutual exclusion, event notification, and thread synchronization features. Major distinguishing technology characteristics of ThreadX include preemption-threshold, priority inheritance, efficient timer management, picokernel design, event-chaining, fast software timers, and compact size. The minimal footprint of ThreadX on an ARM processor is on the order of 2KB"

ie it's generic code that gets built into a unikernel-style image with the device-specific code that actually implements the various tasks required of a wifi controller.

Embedded micro controllers are very common these days in a wide range of ICs most of them aren’t disclosed or accessible to their users their sole role is to abstract the physical IC and present what the customer expects this can be things like timers, flash memory or even microcontrollers themselves as well as more complex ICs that need to ensure some defined PHY like network or serial controllers.