It's just a plain z80, according also to Guru^TM readme. Probably there was an old dump that didn't disassemble correctly ... or not? I've always thought that was a z80 anyway, fwiw ...
Probably major issues comes with the VDP stuff ... I'm getting some results now, the BIOS is really complex and I bet whatever you want that without major z80 dasm skills you won't go to anywhere, first thing that comes to mind are the 2 (yeah, TWO) EEPROM devices.
