I would like to see a source for this 1000ft hypothesis
All he's saying is that by the time you react, get the lever down and the aircraft settles into its fully developed auto with a stable rate of descent, it takes about 1000 ft.
Leading up to that, there will be various variables that take a while to settle down, as he's already described above.
That's not the same as saying you can't fly a proper auto from pretty much anywhere up the transition slope, or anywhere really - you just have to judge how much flare and how much lever lowering you can do.
Obviously from a decent height you can get the collective down, flare to achieve your desired speed, stabilise at a constant rate of descent and have a cup of tea on the way down, but from lower down you won't necessarily be able to do all those things. You may have speed that you can trade for extra potential, maybe not.
As you'll note, E86 has mentioned things like the delay involved in your brain processing what's happened, during which the Nr is decaying like a bastard. Hughes500, you obviously got the lever down quick and did what you had to do, but that's not to say everyone would. The other thing to keep in mind is that he's only talking about being established in full blown stable autorotation, not just in a position where you can pull off a landing, if you get my drift.