thenobledrake |
Ghoul, even as a level 1 creature, gains 1d6 healing from consume flesh. It's right there in the stat block.
Ghast, despite being ghoul-based and level 2 creature, gains 3d6 healing from its consume flesh action.
The "for every 2 levels it has" clause is entirely about building non-standard ghouls and ghasts. It is also likely meant to be guaranteed as at least 1 (or 2 in the case of ghast) die of healing because otherwise a significant piece of the flavor of the ghoul/ghast is lost.