Honestly, this whole KVM thread is a bit confusing to me. I've always equated KVM with keyboard/video/mouse switches: enabling multiple computers to be controlled by the same I/O devices. Nowadays I just use a multi-source monitor and USB switch for that (can even automatically control the monitor via DCC, depending on the switch state, look it up on HN).
And I usually refer to the "IP KVMs" as a "lights-out" module/engine as I was familiar with HP's "ilo (integrated lights out) [0]. More on lights out management on Wikipedia: https://en.wikipedia.org/wiki/Out-of-band_management
I was thinking of making my own with an ESP32 + serial, as that's enough in theory, though I am unsure how well-supported serial ports are in BIOS and bootloader. Also, IIRC RS232 is ±12V, so the ESP would need a level shifter.
TFA's device looks cheap enough (well, depending on shipping to EU), I may just get one. The screen is a bit superfluous though, and I think that the ATX power button support is a must.