I'm not sure why 'dimensional' is more clear than 'spatial' but I imagine it's because Apple showed everyone a bunch of 2D planer windows floating in space.

Thinking beyond the initial onboarding of today's 2D designers, they also showed hints of how apps can easily become more 3D, with elements coming out into space. I predict the 2D window aspect goes away over time. But it will take time to convert people from one way of thinking to another.