Even the versions where Brainiac did so had it be the planet's core. Saying Brainiac merely exposed the core or made it vulnerable by stealing Kandor out from above it.
The core being unstable makes more sense to me. In a lot of versions, Brainiac is behind destabilizing Krypton's core. It's more believable for me that a powerful being caused the destruction of Krypton than it being destroyed from its sun.
