Yeah that's what disturbed me most about the ravaged planet. For it to look like that it would have to be a geologically dead and cold planet like the moon. I would have expected it to look more like that
,except with the left side still intact. The way they depicted it made it look more like a joke. I guess the bomb somehow "cauterized", flash-cooled or whatever the liquid core at it's boundary and the rest of the planet didn't have time to go through any changes yet.
As the bomb doesn't explode and deposit any energy into the planet but rather simply made anything it comes into contact with to disappear, it would explain the latter but not how there's no magma especially as the bomb reached deep into the mantle. The sudden loss of pressure from all the upper layers of stone vanishing would have meant that the magma would have violently expanded. Now depending on if the pressure change already occurs progressively while the bomb is still active, it could happen that all the ejecta vanished as well and the rest of the core cooled down accordingly. If the pressure change happens instantanously when the bomb imploded would mean a plume of magma shooting out of the planet, sort of like a volcano the size of Gallia 4.