I'm actually the guy (Sam) from that post! Never got DOOM running on the Furby, but I was able to get a custom version of Pong running, with accelerometer controlled paddles.
The CPUs are definitely weird, and I ended up porting Ghidra to support u'nSP as part of that project. https://github.com/SamuelWAnderson45/sleigh-unsp Reverse engineering a CPU was a wildly fun learning experience; still proud of that project.