Even taking into account the fact that its is "parable" and not recount of the history of Git (which you can find in some detail on Git Wiki, by the way), one point stays: it is IMVHO bad practice to explain staging area in the terms of splitting changes into more than one commit and/or comitting with dity tree, i.e. with some changes uncomitted. Staging area main strength (besides being explicit version of other SCMs implicit to-be-added area) is dealing with CONFLICTED MERGE, and that is how it should be explained, I think.