[Kanon]

Spoiler for 234:

The characters are aware Sukuna is keeping an ace in the hole, and it's the reason they're not joining because it's a completely unknown quantity. Pretty sure he has more techniques than the ones he's displayed. Flame alone would never allow him to turn the tables.

I'm thinking he has a "vault" full of techniques he stole from other Sorcerers, hence the "open" when he used the flame one.