The main difference is that in “Where’s Rebecca Bunch?” those characters had individual verses so it was an actual group song type performance much like “No one else is singing my song.” With the kaorake Song it’s very much just a duet with Maya and Nathaniel performing all the verses and the background characters just singing along.
By that logic we should at everyone that joined in including female bar customer, male bar customer, law firm assiciate #1, law firm associate #2 etc etc. Greg was simply joining in the refrain along with a bunch of other background characters. It’s not just because they sing in that song it’s if they were featured prominently during the performance or had a relevant verse. This song was clearly a duet, two people Maya and Nathaniel.