How do they know that the structure has only negative curvature and not regions with positive curvature? (like a bunch of soap bubbles that have coalesced)The s

How do they know that the structure has only negative curvature and not regions with positive curvature? (like a bunch of soap bubbles that have coalesced)The structure in the paper labeled ztc-RWY certainly looks like it is ONLY positively curved.