I'm not mad at all, so if my post read like that, that's not how I meant it.
Mostly I'm bewildered, because this is Glee.... The reasons for this being a DS (too grandiose, Santana and Rachel being there etc) seem so par for the course for Glee that I just don't get it.
This is the show that skips all logic to have its way. They wanted a huge production number for the season opener, so that's what they have. No further logic required on this show.
Not everyone pays as close attention to spoilers as we do here. And glee has been known to do dream sequences on many occasions. Given that and the grandiose nature of this particular performance, I'd say it's understandable.