not necessary FPGA but can be re-writable: see
I am not good in that hardware emu branch but my guess is that they pick something that can drive and matches original clock speed as the old programmable rom was no longer produced. (the antique people would buy old broken ones and rip parts off them or try to restore them.)
Thanks for letting me know about this logic table thing, that explains my question when younger why some old computers had massive array of same components put together.
ps. my first computer was a 80286 knock off. By the time I get to high school(basically 80386 era) that have a computer tech club where member bring their old computer parts to share, they are mostly no longer functional. I basically donated my old 80286's 20MB hard drive for tear down and that's first time me and other member see what it looks like inside a hard drive.