tl;dr:"DX is about developer feelings – it is a sociotechnical system which should consider every touchpoint a developer interacts with to plan and produce software, from learning a platform to the very first line of code all the way through its promotion into production."
tl;dr:Progressive Delivery is the term the author has coined for the new basket of skills and technologies concerned with modern software development, testing and deployment i.e. Canarying, Feature Flags, A/B testing at scale.