I think what they're saying is that larger radii are for 'real windows' that have toolbars and such but there are 'mini windows' and those get smaller radii. It doesn't seem well enough baked for them to release it like it is but there are other UI problems that I've been annoyed about for a long time (in particular shadows around window boundaries so you can never get a truly flat tiled experience).

Rounded corners (and the utterly massive drag area next to them) are touchbar 2.0. Features that no one asked for, has questionable value, and that provides marginal benefit even for its intended audience (touchscreen macs, no doubt).