That's why the XP book arranges itself into values, principles, and practices. The best line in the book is about how practices without underlying values are dead, while values without practices are wishy-washy abstractions. What he's really advocating for at the highest level is skilled teams, who are given ownership, that are actively defining their own processes, and executing them with discipline to produce well-designed and reliable software. The book is a "grab bag" (very legitimate point) because those are the sorts of techniques that those kinds of teams use.