While the blog does use "primary", people are saying "master"/replica, much like how redis refers to their architecture[0]. In fact, ctrl-f "slave" only brings up your comment.
It would be better if we completely abandoned that terminology.
1: https://news.ycombinator.com/item?id=31341392
2: https://news.ycombinator.com/item?id=31341661
3: https://news.ycombinator.com/item?id=31340318