A completed GXEmul implementation of the Iyonix would provide the equivalent of RPCEmu or VirtualRiscPC emulating an Iyonix on a PC, Mac or another suitable platform - useful for running RISC OS 5 or testing ROMs from ROOL source before you write them into your Iyonix Flash ROM.
David's work is not a machine emulator. It will run on ARM-powered devices and sits on top of another OS, such as Linux. It tricks simple programs into thinking they are running on RISC OS when in fact, they aren't. It translates ROS system calls into Unix system calls, so the underlying OS does the real work. Effectively, it allows you to run ROS apps on ARM Linux.
Timothy's work, mentioned above, is similar to David's efforts except that it'll run on Intel processors, allowing RISC OS programs to be tricked into running on a PC with the help of a built-in ARM emulator. When/if released, it'll allow you to run ROS apps on an x86 machine.