That’s not true. Tube framed aircraft are made of steel as well as wing struts. That’s why unsealed struts have to be hot-oiled regularly.
Some parts are optional like wheel pants so there’s a ready made set of attaching points right there. It would be really easy to mount a go pro there. And most planes have landing lights so there’s another excellent place to mount a camera. Pull one of the lights and screw it right in. Some low wing planes have plastic visual tabs on the end of the marker lights so the pilot can see if they’re on. Pull that off and replace with a mount.
The rotating beacon is another good spot. You could use a worm clamp to secure a camera front and back.
Those are a few places I can think of off the top of my head.
Oh, and inspection plates. They’re all over the plane. remove one and install and screw in a camera.