To make a bike "light", you need to concentrate on all components; the frame is a relatively minor part. eg in road bikes, the frames are tending to 1kg and less in a bike that is 9kg.
A folder needs stronger tubing due to the lack of diamond frame to make it stiff enough. So the frames are already heavier than for road bikes. But the bulk of the weight still lies in the other components. It's just that the frame is the largest component and schews a person's perception.
The wheels are one of the major parts weight-wise. Rims, spokes, hubs and tyres are all heavy parts. So there is the answer. A Strida weighs so much because it is a ridable bike which doesn't cost the earth.