The show's creators always said that LOST was planned from beginning to end - but in a way that was flexible enough to adapt to new ideas and circumstances along the way without sacrificing the Big Plan.
The thing is, for me anyway, I just I don't believe them. I'm kind of an asshole in that if you lie to me once, I'm very skeptical of everything you say from there on out. In their case, the "Oh this is not a sci-fi show, everything can be explained through normal science" bullshit line has made me VERY distrustful of anything they say.
They may very well be telling the truth about having it all planned out, but I find it hard to trust them.