I apologize , You simply just misunderstood the importance of the concert, then
It is a personal storyline for Jill, but it's not an important thing for MC and Jill Relationship. You can help Jill regardless if you are on romantic path or not.
And since it's also an optional thing for player, whether they actually click on the guitar and practice that song or not, it's not a deciding factor. Jill will have sex with MC even if she not perform.
so overall, it is a bonus rather than a milestone, unlike the "picnic ", which is an important milestone, that lead to the next step.
of course the Dik affinity is an another thing, that is a nonsense, for sure.
but even that, is not about the concert, but, about that Jill looks at the moon and decide that something is off with MC . and You automatically friendzoned.