What other MCU ICs have been demonstrated to use on-die PIO to "not" bit-bang 100BASE-TX Ethernet?

Notably, the XMOS xcore.ai series of chips have no dedicated hardware peripherals - their 100BASE-TX MAC (as well as UART, SPI, I2C, I2S etc) is entirely software defined. A very different approach to most other microcontrollers: https://github.com/xmos/lib_ethernet

I think a more prudent question is "why not use a microcontroller that has the interfaces you need?"

STM32s with Ethernet are cheap and available, I don't see the point in gymnastics like this

Obviously they are testing the limits, I think most people understand that just because they prove something is possible they aren't saying it's a normal approach.

Even then, pi pico are dirt cheap and have all sorts of features. Reading from i2c or sensors then putting it out over ethernet could be very useful.