I don't know what that mod you reference does underneath it all but if it is anything like the different elder scrolls in other elder scrolls efforts (and modern fallout and modern elder scrolls are the same engine underneath it all) then most of those are closer to full porting efforts than some kind of hypervisor, object import or compatibility layer like we sometimes saw for old Doom engine efforts. Persona games also feature fairly high levels of scripting/scripted events which the engine could be made to handle but might also be custom code (not necessarily the best programming method but quick, cheap and easy, and more than acceptable).
I would also be careful with the word decompile. For some it will reference the act of turning compiled code back into high level source code, see Diablo, Mario 64 and now Zelda 64, with many other things coming along nicely. It could possibly be done; late PS2 era maybe still seeing enough C (as opposed to C++ which is presently extremely tricky/basically unworkable for this) to make that a useful notion.
For others it will more simply be the act of unpacking the ISO/exploding the ISO into all the component files so you can poke at them with a stick and see what they are made of.
