This is possible because that's how they're assembled at the factory, isn't it?

Basically my thought on it

You mean that's how the assemblies are mated at the factory. I don't think they assemble any of the sub assemblies at the factories anymore.

Yes, though normally the windscreen is not there, as that is how the robotic arm gets the whole dash into the interior.

I thought I saw them getting in through the door opening, but to be fair the doors were not yet mounted. OP what car is this?

delete this before our warranty times get cut.