I'm not sure if you are actually disagreeing with TSG.
Dawkings repeats several times in the book that to him evolution is not about individuals, but about genes. Heck, even the title is "selfish gene", not "selfish tortoise".
Your tortoise benefits from screwing over his competitors, but his genes might not. Like, for example, suppose there is a combination of genes for noncooperative tortoise. He leaves others to die so the next generation is all assholes like him and they die out because there is noone to help when they inevitably get in trouble. Boom, genes are gone.
In fact, TSG also tries to analyze how cooperative and noncooperative strategies may coexist in single species and tend towards some equilibrium, which depends on costs/benefits of particular behaviors. And, iirc, such crude strategies like "always defect" often ended up outsmarted by subtler ones.