I'm not surprised that it worked since it is my understanding that from the start the
Freedoom project was intended for running mods and TCs.
It's also very similar to the way the DOS full version of
HacX (v1.1)
can be run without the original DOOM2.WAD.
HacX v1.2 uses assets from
Freedoom in its main IWAD to substitute for the files (like the main palette) that it would originally read from DOOM2.WAD.