How much understanding is needed? If it works as specified (you did a complete spec and occasional check-ins to ensure things are on track and then tested the delivery, right?), then what more is needed?

Ah yes the Santa dev fallacy.

Noone understood the code to begin with. People understand the concepts, the primitives, the "theory" as Naur said, the basis upon which a project is built. Noone ever understood a codebase worth understanding to begin with.