That seems like double-counting, then. The density numbers are based on the total area divided by the total residents. If the residents are squeezed into half the area, it should not be said that there is lower density and more space to spread out. The lower density is already because of the space.

I don't think so, the point there is that SF physically has no room for suburban sprawl because it's a single stranded peninsula. Berlin is both currently less dense, and also has further room to expand geographically. One might disagree but it makes sense that way to me