Maybe you're right but...
I had a personal tour of the SR71 at Boeing's Museum of Flight. A retired engineer who was guiding some unruly brats left them when he saw me staring at it and walked the entire plane with me. He a had a pointer like the ones grade school teachers used to use. We got to the leading edge and he rapped the underside of the wing, 'tink' 'tink' tink' "See, that's titanium" he said. Then with a twinkle in his eye, he hit the leading edge...'Thud'. "That's composite..."
I found a couple of sites that mention non-metallic materials. This one seams credible and a very good read:
The U-2's Intended Successor:Project Oxcart,1956-1968
"On later OXCART models the fillets were made from electrically resistive honeycomb plastic with a glass-fiber surface that would not melt at high speed. When struck by a radar pulse, the composite chines tended to absorb the pulse rather than reflect it. A similar approach was used for the leading edge of the wings."
RobVG