Something else I thought of, too - which is faster, to context-switch and take advantage of the computer's help, or to just carry on typing? If you're a fast typist, the computer "being helpful" is just getting in the way.
I play classical guitar, so I actually class myself as a 6-fingered typist, and typing this comment here it's actually quite obvious. I'm using my left hand "correctly" as a touch-typist and it's making almost no mistakes. My right hand is doing a two-fingered "hunt-n-peck", and making a lot of mistakes. So even if I don't have something to copy, autocomplete can be a real nuisance for me.
So yeah, for many of us auto-complete probably is helpful. But for someone who knows how to type, it's almost certainly faster and easier to turn it off (that is, you can find out how!) and do everything long hand - you'll do it faster than you can context switch.