Apesar de os fãs terem ficado confusos durante anos, Wikipedia confirma que era a voz dele, mas não é ele cantando .
For contractual reasons, [Jackson] was credited as John Jay Smith in the closing credits, and his role in the episode was not officially confirmed until later. Jackson was a fan of the show and called creator Matt Groening one night offering to do a guest spot. Jackson pitched several story ideas for the episode and wrote the song "Happy Birthday Lisa" that is featured in the plot. He also stipulated that he would provide Kompowsky's speaking voice, but his singing voice would be performed by a sound-alike (Kipp Lennon) because he wanted to play a joke on his brothers.
The producers agreed [to Jackson's terms], although as it turned out, they were so annoyed by having to evade questions about whether it was actually Jackson doing the voice they made a rule that, from that point on, all guest voices on The Simpsons would have to be willing to be credited under their actual name.
Just to add extra confusion to the situation, when it came time to record the episode, while the impersonator, Kipp Lennon, performed the songs in the episodes, Jackson decided to also record the songs, just for fun. As a result, there has always been some murkiness, even among Simpsons staffers, as to whether any of Jackson’s performances were used on the final show. The Simpsons music editor Chris Ledesma, however, has confirmed that it was only Lennon’s performances used on the actual show.
Aqui está Dan Castellaneta (Homer) discutindo o envolvimento de Michael Jackson no episódio: