Frictional Games talks about their secret project

[the dark side]

well, contrary to popular Beleif, developement isnt just the programming, the programming is actually phase III of development, Phase 1 is the "pencil and paper" stuff the outline, the basic plot comments, etc, the overall genre, phase II is stuff such as storyboarding, research, Concept Arts, level Map ideas (pencil and paper layout blueprints) etc.

so they could have started development, as in Phase I pencil and paper game outlines, before Amnesia was completed, as i said, programming is phase III of development, a part of the process, not the whole process like most people think.