I don't see anywhere that the author is suggesting that deadline must be explicitly defined up-front, or even be fixed. If you're writing something, it _will_ be done at some point (or is expected to be).
His other writings on the topic are very explicit about setting deadlines on every project and goal. Though, I don’t fully agree with the GP. Thiago also advocates freely changing deadlines as desired, so the deadlines become more a scheduling and prioritization helper than a typical deadlines.