Of course it's another universe. So is every other TV show, movie, book, short story, fantasy sports team, etc. ever created. BSG isn't "more" another universe than any of those.
"Fictional Universes" do come in varying degrees of realism. You can have a really far out alternate reality like in Lord of the Rings
, or you can have something that's simply a fictional story with fictional characters that are grounded in our world like Law & Order
. They're not all the same. Now the question is, how close is BSG
to the latter.