What code are you looking at?

FFIX for PSX would have been written in C (or possibly C++) with PSY-Q. It will not be one program - those games were composed of multiple overlays that are banked in / out over the PlayStation's limited memory.

From what I know the PC release was a port to a new framework, which supports the same script engines, but otherwise is fresh code. This is how it can support mobile, widescreen, Steam achievements etc.