This is just a matter of you not being caught up I think. Also some blatant misrepresentation of events going on

1. Sunny resisted Nothing for 3 reasons.
A. The Nothing in the hollow mountains is far weaker than what surrounds the flame as it is said to be mere wisps that leak in.
B. Sunny’s incarnation in Ravenheart anchored his sense of self allowing him to retain his sanity.
C. Sunny was empowered by the flame of desire, something that at the very least is just as if not stronger than, Nothing.
2. Nothing does not seal the Void. It merely affects the Void Beings. Beings who are not empowered by the flame. Even then there are Void Beings who broke through this cage and entered the realms regardless. For example the Void Being who shattered part of the Shadow Realm in the “Fragment of the Shadow Realm” runes and the Void Being who was killed by the gods in the hollow mountains, from whom True Darkness originates from.
3. Nothing simply separates the Void and the flame. And it was used by the Gods (avatars of the flame) to create a barrier between the flame of desire, after which the flame entered a self sustaining cycle of creating and burning its own fuel.