current source code provides support inside world_gen.py, this however can't work since it uses the tag "antiqueatlas" instead of "antique_atlas"
this is how it shows up in game:

this is a easy fix, please fix it
line 1601 in world_gen.py
rm.data(('atlas', 'tiles', name), { 'version': 1, 'texture_set': 'antiqueatlas:%s' % atlas_texture }, 'assets')
potentially fixed version:
rm.data(('atlas', 'tiles', name), { 'version': 1, 'texture_set': 'antique_atlas:%s' % atlas_texture }, 'assets')

i think some of the biome names for antique atlas have also changed

turns out, the problem might be far more complex than this, not even making a resource pack and adding all the jsons in the correct place, or replacing the ones inside terrafirmacraft fixes this

To paraphrase from the (long) commit message, as Antique Atlas is not updated past 1.18.2, past an unofficial fabric port (which works on Forge through connector), making it both undocumented + impossible to setup in a dev env, the existing compat in 1.20.1 has been removed.