They are significantly less rigid than a properly hardened hook and are made of finer wire than most sacrificial hooks used for articulating. So, yes they are lighter than the hook they are replacing.
Looking at the fine bending jig posted earlier you see that he is bending the wire and it is staying put after bending... Imagine a hook bending that easily... and not breaking. Those light wire shanks will be fine for smaller fish and will not likely fail during a fight with a big fish. But you will likely be retiring the fly after every decent fish.