Wannabe 30 Rock expert here. There are a lot of misunderstandings about the extent of Glover’s involvement in this song. Here is a quick recap of the timeline:

1. The song started as just two lines in a quick throwaway joke in an episode of the show. These two lines were written by Robert Carlock and Tami Sagher and performed by Morgan, with Jeff Richmond creating the backing track.
2. That joke was immediately a huge hit. NBC’s website had a clip of that joke, and that video had a large number of views. The writers decided to capitalize on that popularity and scrambled to create a full version of the track before Halloween, which was coming up in just a couple weeks. Sagher was tasked with writing the lyrics to the full song, and of course Richmond created the full backing track.
3. They were able to get Morgan back to record the new lines that Sagher wrote. This is also the point at which Glover finally got involved. He recorded a number of spoken lines in the song, portraying the character of the song’s producer.
4. After recording, while mixing the final track to get it ready for release, it was determined that something was wrong with the recordings of a couple of Morgan’s lines. They needed to have Morgan rerecord those lines, but he was not available to do so in time to meet their Halloween deadline. So, Glover stepped in and rerecorded just those couple lines, using his best Tracy Jordan impression.