Hmm, I don't think I agree. Really looking at a high res version of the concept art it's clear the building you guys are cutting off is the same one that people are walking behind, in the concept art. You can see the obvious plaza full of people, but also people cutting behind the building as well.
It however looks like the corner in between the final orange building and MF will be back stage access, obscured by a big pillar.
Of course that all comes down to how much at face value are we accepting the concept art. In land "overview" case it looks pretty accurate. I also just don't see why they'd make one of the 'new' backstage buildings orange but the other one that is far more permanent with steel unlabelled... although still somewhat visible on the master plan.
The other option is the warehouse remains "backstage", but it gets all that facade in front of it, just with no actual content accessible behind. Which now that I think about it might actually be the case. There is a giant closed door in that corner on the concept art.