Can you define “non exploitation”?

I probably could, but I cannot say what Donald Watson or Leslie Cross or anyone else meant.

My understanding is that leather is a waste byproduct of the meat industry

It is a co-product, and it directly supports the industry.

so much in the same way that gas is from dinosaurs that are already dead, the cows that provide leather are “already dead” due to their use as meat.

Dinosaurs did not die as a result of humans exploiting them in order to consume their bodies. Cows live and die solely for the sake of exploitation.