I probably performed a Google search for "Bitcoin forum" and made it here through one of the results that popped-up.
It was pretty much the same with me when I was first looking into what bitcoin is. If I remember correctly, the "other bitcoin forum" came out higher on the Google search results back in the day hence I joined that first, also thinking that it was the "official" bitcoin forum lmao. I then realized after a while that Bitcointalk was far more active.