Seriously... they're not trying to make out that there was any grand "Plan". I don't know where you're getting that idea - from the title? The movie itself shows that there was indeed, a plan...(well, duh... there always is one...) a very simple and not very good one, to destroy all the humans, and when it - of course - did not work (did they really think they'd succeed in wiping out 100% of a population at one stroke?!) Cavil just started improvising and coming up with desperate little "plans". In other words, the movie itself mocks the idea that there ever was some kind of grand "PLAN" and that the Cylons were oh so knowing and powerful and that they had everything mapped out, as people used to assume during season 1 and most of season 2.
Or, if you like theobabble (which the movie was refreshingly free of, and I find it a great relief), you might take the words from Cavil's poster literally despite himself, and postulate that the only big "plan" there was was "God's plan".
If there really was no "grand plan" ... then why the hell devote nearly two hours to the ad-hoc, ridiculously incompetent and futile plan Cavil came up with ... and call the whole ball of wax The Plan
? Unless the retcon was worth exploring, why do so at all? Why not just leave the mystery intact? Afterall, the mystery of Vader's past turned out to be far more tantalyzing than the reality.
If you're going to go through the trouble of explaining "The Plan" and call the movie The Plan
... then "The Plan" should be at least something more compelling then: KILL ALL HUMANS, INCLUDING INNOCENT HOMELESS BOYS ... <EVIL GLARE> GRRR!!!