Sort of. At the time, that was the last episode filmed before they had to shut down due to the 2008 Writer's Strike and SyFy told Ron Moore that if the Strike went on for longer than a certain amount of time, they were just going to pull the plug and that would have to serve as the series finale. Fortunately, the Strike didn't go on that long, and they were able to resume production and do their own ending.
Well, not quite the last episode filmed. From what I understand, they shot all the groundside scenes for "Sometimes a Great Notion" before the writer's strike (at the same time they did the one closing shot in "Revelations"), but all the stuff in the studio after they came back. I expect they shot at least one alternate version of a scene while they were on location, just in case the delay changed their plans, since before the writer's strike, it was expected that Lucy Lawless was going to be recurring in the second half of season 4, but I guess her availability changed, leading to D'anna's somewhat anticlimactic decision to sit quietly and wait to starve rather than continue to put up with the tiresome rigamarole of not being dead.
Likewise, for reasons I'm about to elaborate on, I am morally certain that if they were suddenly, conclusively cancelled before "Revelations" broadcast, and Ron Moore didn't decide to utterly destroy the story he'd been telling out of overwhelming spite, they would've trimmed the last minute from the episode and just left it ambiguous what they found when they got to Earth.
I don't know, that feels like a much more honest ending than what we got...
I disagree. The blatant optimism and unreserved sentimentality of the endings of (checks episode list) the miniseries, 33, Water, You Can't Go Home Again, The Hand of God, Home (Part II), Flight of the Phoenix, Resurrection Ship (Part 2), Scar, Downloaded, Exodus (Part 2), The Passage, A Day in the Life, Faith, Sine Qua Non, and The Hub, among others, established a clear dramatic and thematic through-line that the characters would ultimately succeed in their quest despite overwhelming adversity, and that overwhelming adversity would not, in fact, win out.
Stories are machines. They are not just a bunch of stuff that happened, they are constructed on a logic and framework which provides the audience with tools to understand and contextualize the events in their own lives rather than being overwhelmed by the sheer random chaos of an all-but-infinite universe and the conflicting actions and motivations of hundreds of other humans in your social sphere, each of which has their own complex internal life. In a thousand ways, big and small, Battlestar Galactica tipped its hand from night one of the miniseries that
this is a story where the heroes make it. I cannot for the life of me understand anyone who watched the show and thought for one minute it was going in any direction other than the promised land. The only thing more baffling was the people who were sure the humans were going to intentionally and overtly counter-genocide the Cylons with no survivors.