It's funny how manga editors within actual manga are stereotyped to be omniscient and possessed of sagely wisdom like this, being able to accurately tell exactly how long he worked on and how much effort was put into his art is quite a feat, especially considering that he had help.
Honestly just have the editor tell the MC that he thinks his art sucks or his plot isn't quite all there, you know ACTUAL CRITIQUE of his work, and not of him personally. if the MC has to learn to not procrastinate, don't just have him told outright through an arrogantly condescending semi-clairvoyant editor, have him learn through his experiences of failure instead. Kokonotsu doesn't need this sort of lame psuedo-guidance counseling session disguised as a professional interview to grow as a person.