And dont forget the entire possibility that Earth was the original cradle of humanity and they moved from earth to Kobol.
I don't see how they could say anything else without denying over 150 years of scientific discoveries and research into the human past. That would turn BSG from Science Fiction into just fiction (insofar that it isn't already)
imho bsg history probably went a bit like this:
1. humans leave earth and colonise Kobol (and maybe other places)
2. humans on Kobol create scientient machines. Basically the first "Cylons". Something happens that leads to a massive upheaval on Kobol and the departure of the 13 tribes. 12 to the new colonies, 1 back to Earth. But whatever the disruption was you can be sure that the "Cylons" have something to do with it and that the difference between the two groups is their relation towards these artificial humans.
3. the 12 colonies settled, somehow the Final 5 remain undetected in the populace. Either by virtue of being well hidden, or the relation of those colonists towards the artifical humans.
4. on the 12 colonies intelligent machines are created, the Cylons. This is the beginning of that part of the prophecy "what has happened before will happen again"
5. Cylons rebel, hybrid construction begins
6. BSG season 1 starts.
Now somehow those 5 old models have managed to set a rerun of history in motion, either by nudging the colonists to produce Cylons or by tampering with the Cylons themselves so that they'd eventually try to build hybrid models of their own. In the same vein it then becomes entirely possible that it were the socalled final 5 that wrote the book of Kobol in the first place.
An alternative possibility is that the 12 colonies are the Kobol Cylons, all hybrids, all imperfect, all mortal, which would explain why after 1000s of years they've fogotten about the 5 models that are actually immortal. But something in their collective memories-instincts gnaws at them. Something that leads them to produce Cylons in an attempt to put things right, but while trying to do so making all the (same?) mistakes the humans of Kobol did. The 13th tribe would then be Humans that went back to earth.
Not that it matters as we'll probably never know.
And didn't I see two other battlestars in the old teasers for season 4 (Hades and Titan?) Where are they?