You're right, it's a proof of concept: "The extensions, each measuring several metres, will replicate a folding wing in the fully extended position during a flight. They will be fitted with comprehensive testing equipment to capture behaviour in flight and evaluate how the longer wingspan impacts aircraft handling."

Not just the shape, but also the mass placement and rigidity. I explain this in another comment, I don't want to cut and paste redundant info here but keep scrolling and you'll find it ))