You have the upper hand with familiarity of the code base. Any "domain expert" also necessarily has a head start knowing which parts of a bespoke complex system need adjustment when making changes.

On the other hand, a highly skilled worker who just joined the team won't have any of that tribal knowledge. There is a significant lag time getting ramped up, no matter how intelligent they are due to sheer scale (and complexity doesn't help).

A general purpose model is more like the latter than the former. It would be interesting to compare how a model fine tuned on the specific shape of your code base and problem domain performs.