It's a bit of a surprise move given the extra story content in Golden was supplementary and not particularly substantial, but let's take it in context. Atlus is releasing a dancing game spin-off, a dungeon crawling spin-off, and another version of its fighter spin-off for Persona 4, all in the lead-up to Persona 5. Maybe, in fact, it makes loads of sense.
Update: The official site is up, along with the trailer.