The message from Rocksteady Studios is rare in that the developers have largely been silent in recent years. Batman: Arkham Asylum may only have been released 10 years ago, but Rocksteady has been working on its current, as-yet-unannounced game for the last four of those years. For Rocksteady to come out from its shell perhaps implies how much the anniversary means to the studio. Even if it was understated.
It's also understandable why Rocksteady's message is surprisingly understated. The 10th-anniversary celebratory announcement could have been bigger and flashier with ease. After all, fans adore the Batman Arkham series. However, the studio is already being watched from all sides as fans speculate about what Rocksteady's next game will be. Rocksteady likely chose to only put out a small reminder so that fans wouldn't leap to the conclusion that Rocksteady is working on a new Batman game.
While it's a shame that Batman fans can't celebrate the franchise's 10th anniversary by looking forward to what Rocksteady's next franchise will be, it's just a matter of time. Early reporting indicates that Rocksteady's next game is a next-generation title with lofty aspirations. Rumors are swirling about a 2020 launch, but 2021 or even further could be possible. All eyes are on The Game Awards 2019, of which Rocksteady and publisher Warner Bros are often participants.
For the time being, however, it remains the era of the Batman Arkham games.