What I'm reading is that Slack underwent a critical period of user-focused product design. This is the story of a product development process, not a launch.
The other factor is hidden behind some of the headings: “When key users told us something wasn't working, we fixed it — immediately.” That's nice to say, but you don't just say that, you have to achieve it, and that's the hard part.
Both of these point to critical internal systems that are set up extremely well. Someone or several people inside Slack know the importance of systems even at their small size, maybe even learned from Deming and TQM, and it's clearly greatly amplified their ability to take all of the user input, product feedback, and vision and distill it into something that's actually cohesive. This probably also applies to their UX process, product design process, and feedback cycle, as revealed by the article. All of this has to be driven by a management style that enables it, understands the organizational psychology behind accomplishing a quality product, and leadership that knows the whole is important. It doesn't happen often.
What you're seeing is the surface analysis of a reverence for systemic quality and the culture, systems, and management style needed to sustain it. It's very clear because of the results. It's like seeing just the light spectra of a star and being able to determine its size and age and chemical makeup; the unmistakeable signature of the inner core.
This would simply not be possible without the ongoing product process that they've set up, and the article is just the tip of an iceberg that lies mostly beneath the surface, large and cumbersome and meticulously designed. Read between the lines and think about why all these efforts have been successful. Take that deeper analysis and learn from it.