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Machine time for something this big is very expensive: that's why it's used for making molds, which is a fabrication technology that scales up very easily; unlike CNC machining, which doesn't scale at all.

Additionally, surface finish with ±0.2mm X/Y is going to be pretty poor. It won't produce finished parts, you'll need to sand/grind it, or coat it with fiberglass and sand that. It also doesn't mention deflection under load, or feedrate through anything tougher than foam plastic.

which part of any of that prevents it from being used for architectural purposes?

If you can use it for boats and blades, you can use it for prefab panel moulds.

There's a big 3D printer that is very suitable for architectural projects. Check it out:

http://d-shape.com/

They don't seem to be printing houses / buildings with this technology. I wonder how readily it could be applied to that. Imagine setting up a rig on a concrete slab and having a house printed in a week.

CNC is probably less useful for printing houses, it's great for negative molds, but harder when you have overlapping empty spaces. It's more useful to have a 3d printer here -- maybe someday we'll have 3d printers that use concrete foam to create simple prefab homes.