All of these can be introduced much quicker than a 12-page document.
Maybe it can, maybe it can't. I encourage you (or someone else) to
show such a variant of the introductory text, then we'd have something
to talk about, including comparing it with what we have now.
And at least for Magit and Helm, it's perfectly possible to describe
their selling points without first introducing what "buffers" and "mode
line" are. Maybe even windows.
Once again, "selling" is a different document. Describing how to use
these feature without the basics could work for those who already know
the basics, but not for those who don't. Maybe you assume that people
who read the tutorial already know the basics.