You're right, they broke off from someone else who broke off from Luther who broke off from Calvin. The SBC is probably the most correct and Christian of the Protestant churches left but they're still a Protestant church that can trace itself back no further than Luther or perhaps Wycliffe. Period.
Anything that goes any further back is fantasy at best and deliberate deception at worst. Actually, it's not funny, it's sad that people try to develop theories to counter obvious history and replace it with something they think makes them somehow look better or be more sincere. I don't doubt the faith of any of the SBC folks I know but I do wonder why they're hung up on pretending they're from some sort of little group that hid in caves for fifteen hundred plus years rather than admitting that they're at the root just more Protestants. And I spent many years in a SBC or Independent Baptist Church so I've heard all the theories and they didn't even make any sense with I was a kid fifty years ago.
Or, were you saying that the Baptists are Protestants who broke away from the wave of Protestant groups Luther fostered and I've misunderstood you?
This was the theory that Joseph Smith took and expanded -- first the mainlines said that the "great Apostasy" took place in the 1400s-1500s, then the Baptists pushed it back to the 300s, so it was logical for J. Smith to push it back to the Apostolic times -- the next step. The 300s theory didn't have any proof, but was held, so why not say the G.A. happened in 33 AD?