Luminary was hardly as an easy guess. I'm one of the few who was able to do so, but it's not like my reasons were akin to the real ones either. All that was really a thing at that point that we could tell is that the most recent version was easiest to promote overall. Erdrick being a bit of an unknown worldwide has really nothing to base it upon, as he was a planned part of Hero from the start. The only thing that's clear is Luminary was intentionally catered to in the product overall, but we don't need jack-all about the development or plans around that. Maybe Erdrick was thought of first. Before we knew Erdrick was a legitimate base costume, there was only evidence overall leading to Luminary intentionally being the star from the start. But details were given more fully. At some point, Sakurai may fully go over it. It could be simply SE wanting the latest one pushed, but keep in mind they're the ones who suggested more than just those two, which is funny since they're known for trying to keep it so Heroes can't be with each other in a crossover, making Smash unique about it.
Sure, the possibility exists(that somebody figured out all 4 alts), but it's not common for people to think beyond the leaked talk. I mean, it's the most talked about stuff. It's an influential thing. Going with the only knowledge one knows is logical. To be real, I only figured Erdrick was one of the costumes, and that Eight or Luminary was the default. Solo I didn't really know of anyway, and I had no real guess of how many costumes we'd have.