The Bane Knight information says that the spell DC for Bane Knight spells is: 10 + spell level + (STR modifierx9/10) rather than 10 + spell level + ability modifier + feat modifier.
I'm guessing feats aren't used for the DC.
The Bigby spells all use an attack roll that uses the caster's ability modifier. The documentation never says specifically that (STR modifier x9/10) is the ability modifier but I'm guessing that the attack rolls should use (STR modifier * 9/10) wherever ability modifier is used.
It would be a bit more clear if there was a line that said that the ability modifier is (STR modifier x9/10).