Providing API for building applications that have transactions and help with idempotent producing is really exciting. This will help lower a lot of pains associated with building stream processing systems. Doing it with very little performance overhead is amazing.
I do feel that calling it "Exactly-once delivery with kafka" is slightly misleading as this requires the applications to be written in a certain way. The title makes is sound too general and borders on claiming something that is close to impossible. I dont want to be too critical here as the author was very honest with what this means in the blog post. Regardless of the title this is an amazing feature.