None of that is documented anywhere else, as far as I can tell. The only startup doc I could find was rather Linux-specific and missing a few things. Anyway. Now my fellow Mac users know how to get started with AO3 development, should they wish to.
My fork has fixes for the two problems I noted in the writeup. You might not experience the problems at all (say, if you don't use Redis or if your version of Ruby is exactly theirs so you don't trip over the yaml engine issue).
ETA: And because no post should be without something amusing, here! Have the 1944 version of The Lord of the Rings.