> a one-dimensional scanline

And iiuc it's only abt 40% of the line. You have to update it during the line! and sometimes it's fwd and sometimes bkwd so you can't simply let it repeat a simple pattern!

I don't know where you're getting that from. You can indeed leave all the registers alone and it will repeat whatever is there for any number of scanlines until you do change it. In fact, this is what happens if you power up the 2600 without a cartridge - you see vertical stripes, because whatever garbage was in the registers on powerup just stays there and repeats onto every scanline, with no software changing it.

"Sometimes forward and sometimes backward" would only apply to the three registers of playfield bits, and is fully controllable by software, in the CTRLPF register.

I'm sure they're talking about manipulating a single line and the fact that you're extremely limited by the behavior of machine for that particular line. Anything that doesn't fit within the hardware design constraints needs mid-line changes and/or clever hacks and, even with those, what you want to output might just not be physically possible.