I've looked in the Config & could only find an entry to disable the Ender Dragon to not be infernal.

Is it;
"WitherSkeletonEntity": true,
To false?

?
~ Gbergz

Yes. That should be the technical name of it. It most definitely is in the config list.

Entries are added to the config as they are first encountered.

Alright I disabled it but it still spawns as an Infernal. I did the testing.
Not sure if there's an entry to disable The Wither as an Infernal.

Ignore this, i added this entry to the Config; "WitherEntity": false, & it's now solved they do not longer spawn as Infernals. This wasn't there from the beginning just letting you know.

EDIT: AGAIN after regenerating the config its there. Sorry lol