He's been talking about this since last year in veiled terms. Maybe longer, I don't recall off-hand when his first public mention was (hell, we probably talk about it and are all "boy who cried wolf" about it in this very thread, a hundred pages ago). As for the relatively short promo cycle, studios have learned, especially after the lockdowns, that long lead-ups to releases aren't as effective at building hype as they used to. With so much being released all the time, a lot of early trailers and making-ofs while still in production like we used to get is now likely to be buried, and by the time the movie or series is actually released, a lot of people assume it had already come out and they'd missed it, and decide they'll just catch it on streaming if they hear it's any good. That can especially apply to projects like this that have long production cycles and hard release dates (in this case, it's a 30th anniversary tie-in). I remember the last nuBSG spin-off, Blood and Chrome, was done for like a year before we even saw a leaked trailer.
IIRC, either the release or a trailer or something was supposed to coincide with SDCC, which is in July, so a first formal announcement just over two months prior feels like a standard amount of time to get the ball rolling.
I had completely forgotten about that 'working on something' post.... I stand corrected.