This project is currently shrouded in all of the mystery that one would expect a J.J. Abrams-produced project to be, so having Dorn announce his involvement would probably go against that rule. However, if it were the case, he would follow in the footsteps of Alfred Molina, who recently revealed quite a bit about his now confirmed role in the upcoming Spider-Man: No Way Home film.
There are options for Dorn to appear in that film. If it was still set in the Kelvin timeline, then he could appear as his grandfather, Colonel Worf, who he played in Star Trek VI: The Undiscovered Country. However, to repeat our previous point, it's unlikely that Dorn would have been given leave to tweet about his involvement, so it's a safe bet that this news is not related to the upcoming film.
That leaves the expanded universe, including Star Trek Online and Star Trek Fleet Command. The hashtag at the end of his tweet is '#ad', which is generally required by anyone who is tweeting in connection with a promotion.
The tweet itself doesn't mention anything other than Starfleet is calling him, so to what end or direction?