That means that they must already have the next DLC planned by the time the last one come out. The steam activity could also have been the bug fixes after Vintage. But now, the activity has to be for a future update and DLC. What happens is that the team decides on the next theme to work on after a DLC is released. And they don’t say what they are working on until a week before release.
They most likely had outlined all DLC planned about a year in advance. They probably spent last summer researching the rides and scenery planned for the Studios, Vintage, and whatever's coming next. By last year's anniversary release, work on the first 2018 DLC release may have been well underway...