The context behind this article is that a lot of people (not me) find the innovative thing about Gmail is precisely that it doesn't render email threads as messages built into trees, but instead clusters them into these flat groups (which FWIW I think most people do associate with the term "thread", as you are browsing by thread, not by message). The article then goes through how they came to this new model, rather than merely implementing the thing which every other client does. And like, while you and I dislike this way of using email, it really now is so popular that everyone else feels like they have to copy it as it really is that popular.