Jeez... You had right all alongThere could be registers in the chip set by the ROM (or machine ID) which affect memory timing.
It could run off the system clock. It could have other clock inputs. It could internally generate clocks, although that last one is less likely.
It would be fairly straight-forward, though tedious to identify the oscillators on the board and see if MEMC IC takes input from any of them besides the system clock.
It just took a numpty [me] to go the long way around connecting the dots like a madman with string and pins at a cork board!