Dev diaries are a thing that paradox is known for doing. Sometimes it goes into the behind the scenes, sometimes it's about work-in-progress features, sometimes it is just showcasing stuff that is already done.

It's basically their name for roughly weekly posts about the current and future state of their games.

Revisiting my comment after the second episode was released: Interestingly, they run the same series of content under two different titles. Under https://forum.paradoxplaza.com/forum/forums/cities-skylines-2.1147/ it's featured as "Development Diaries", while under https://www.paradoxinteractive.com/games/cities-skylines-ii/features the exact same bits are labelled "Feature Highlights".
Both places contain the same video, again titled "Feature Highlight", and the same roadmap for upcoming episodes of this series, titled "Dev Diaries Road Map".