One thing that isn't covered is that 3.5" discs (and presumably 3" as well) actually do still have an index pulse, even though there's no index hole. The metal disc attached to the centre of the disc is keyed (the rectangular hole offset from the centre), so the drive generates the index pulse from the position of the spindle, rather than using an optical sensor to read the disc.
It is, but also because it's grabbing something with fixed orientation to the disk, it also means that the disc is exactly aligned with the spindle every time, and so the index pulse can be generated from a cheap electromagnetic sensor on the spindle instead of requiring an optical sensor.
However, it seems you might actually be right in practice. I think early 3.5" definitely did this, but while googling to determine what orientation of the hole triggers the index pulse came up with nothing, except a few patents about how you can fake an index pulse by using a timer and a speed sensor instead.
So maybe because almost nothing was actually using the index pulse anymore when 3.5" drives were introduced because almost every modern system (for the time) had settled on soft-sectoring, it's entirely possible that I've always been mistaken and there's never been a standardised index pulse for 3.5" drives between different drives relative to the "start of the track".
I guess this might explain why even though the Amiga allowed you to read the INDEX signal, it actually didn't use it and instead read a whole track at once starting when it saw a particular sync word.
