So does that little thing just output RGB to the computer's MB to send out?
How did this "computer" stand up to it contemporaries of the time?
Jeez, do we have to do all the reading for you? Following the link reveals:
The Sharp X-1 Twin was a weird combination unit, featuring the X-1 computer (a Z80-based precursor to the X68000) and an integrated PC Engine. I use the term integrated loosely, the two systems shared the same shell, but little else...
This is the PCE riser/interface PCB. It's brown, and does little more than connect the X1 PCB to the PCE PCB. Neither system is in any way aware of the other...
The PCE does not output RGB through the X1's monitor port, it only outputs composite video and stereo audio, via the AV connectors.
No RGB through the monitor, and the X1 was a crusty old thing: Z80 CPU, low-colour non-scrolling graphics. No sprites IIRC. The PCE inside the same box outclassed it in almost every way, except that it had no computer peripherals.