Jimmy has a self destructive streak.
All he had to do was wait a few days to seek approval from the partners for the commercial. When speaking with Kim later, all he had to do was admit to her that he ran the commercial without approval. On some level he has to know she will eventually find out that he let her believe that he had approval. She's now going to be extra upset for both the commercial and 'lying' to her.
If I were a firm partner, I'd seriously question Jimmy's judgment in running the commercial without clearance, even if the outcome for the firm was positive...this time. I too would have called him and chewed him out. When he slinks in the next day, I'd chew him out some more and tell him that this was "strike one" for his job.
Jimmy's not a fit for this firm. It's no surprise he ends up in sole practice.